How is Samson a witness for the Lord?

Samson could be seen as a witness for the Lord, or a precursor for Christ, in one sense. He gave his life to defeat the enemies of God's people in Judges 16:30.

At the same time, several commentators have pointed out that

Samson, David, and many other excellent men, fell into grievous sins. [Luther's Commentary on Galatians]

And the sins weren't merely the sexual liaisons. It has to do with a spirit of revenge. This "Sermon Notebook" states that Samson's acts of revenge are not to be copied. I would assert that his destroying the Philistine's wheat fields is wrong for various reasons, including Deuteronomy 20:19's prohibition of destroying farmland.

So, in another sense, Samson is not a witness to the Lord, except as a negative example. The hymn writer is as likely to be celebrating "self-sacrificial death for one's people (Judges 16:30)" as "doing a lot of bad things to bad people." If the latter were the case, it demonstrates that many hymns may inspire but have to be understood they're not of a "revelation" equivalent to scripture.

I have always thought of Samson as a figure for brokenness and healing. The whole book of Judges goes to show that God can use anyone at all as a witness for him.

I would tell you, but I have not the time:

Hebrews 11:32-38
What more shall I say? I have not time to tell of Gideon, Barak, Samson, Jephthah, of David and Samuel and the prophets, who by faith conquered kingdoms, did what was righteous, obtained the promises; they closed the mouths of lions, put out raging fires, escaped the devouring sword; out of weakness they were made powerful, became strong in battle, and turned back foreign invaders. Women received back their dead through resurrection. Some were tortured and would not accept deliverance, in order to obtain a better resurrection. Others endured mockery, scourging, even chains and imprisonment. They were stoned, sawed in two, put to death at sword’s point; they went about in skins of sheep or goats, needy, afflicted, tormented. The world was not worthy of them. They wandered about in deserts and on mountains, in caves and in crevices in the earth.

The song isn't saying that Samson was a witness because he was fooled. It's just a very short story of Samson, followed by a chorus. The biblical reasoning would be more clear if the last line told of how he cried out to God and was made powerful again so that he could avenge himself by bringing down a temple, but the song already told of how he killed many enemies.

It's telling a story and saying that Samson was a witness of some form. It's not being very specific, but then again it's supposed to be more lyrical than theological. It's praising God through the recollection and praise of certain good people.

Folk songs are about the context as much as the text.

Bruce Jackson writes in the introduction to his book Wake Up Dead Man: Hard Labor and Southern Blues:

There is an important difference in our approach to art song and folk song. Art song requires that we perceive the nature of the art involved; folk song requires not only perception of the art but also the generating or supporting musical social, and historical contexts. Folk song is not simply textual, but con-textual: it does not exist—save for historians and scholars—on pages in books, or even on shiny black discs. It exists in a specific place at a specific time, it is sung by specific people for whom it has specific meanings and functions.

We can't now determine what the original context of the song was nor who wrote the Samson verse. But we do know that it existed in one or more of the places that African music thrived in America: work songs, church worship, and minstrel shows. The most obvious context is Christian and we know the Fisk Jubilee Singers (who avoided vaudeville music) used the song on tour and recorded it in 1911.

Meanwhile, Bruce Jackson also noted that two versions of the song were used by convicts in the Texas prison system. One version repeats the basic refrain and inserts various non-Biblical people into the role of witness: family, fellow-prisoners, guards, even visitors. In this context, the message is clear from a single line:

Anybody can be a witness ...

The other version is an adaptation of the spiritual. Jackson, analyzing the final stanza about a man who caused a stir at church (perhaps a one-man ring shout?):

All the previous stanzas are about situations in which God was involved in people's activities; this stanza suggests that the use of messengers and agents might continue.

The purpose of the song in worship.

As a work song, the purpose is to get by: make a hard day's work go a little lighter and a little more pleasant. But in the context of the church, it must have been something different. Part of the purpose might have been entertainment: the Samson verse fits the bill perfectly. But the other purpose, judging from the other stanza, must have been to remind the congregation of the Biblical stories. Since I wrote the question, I've seen verses about Moses, Ezekiel, Isaiah and Jonah which pick significant moments in their lives to put in verse. One can imagine these verses being passed from one congregation to another (and even back and forth from church to work) and thriving or dying depending on how well they were remembered.

Many of the verses show men sustained by God in hard times. Power to overcome human authority seems to comes from God Himself. Remember that the church was one of the few places a slave could be free to express himself without fear of reprisal—as long as he didn't speak rebellion against his master. So the songs seem to be coded to communicate that God can (and might very well) free people from bondage without saying it outright.

Samson empowered by God; Samson defeated by trickery.

On the one hand, God's hand was clearly on Samson during his time as judge since he was able to easily defeat the Philistines with his strength. But he also used trickery, which was his downfall in the end. When Delilah removed his hair and he became a normal man, it was as much a witness to what God had done as Methuselah living a long life. The song calls it's hearers to be witnesses to the Lord, but it assumes the audience is as powerless as Samson shorn. By implication, God might very well empower someone to become a witness by becoming strong enough to shake off the bonds of slavery and oppression.