What is the use of a blue UN Laissez-Passer?

The United Nations Laissez-Passer (UNLP) is a travel document which designates the holder as a UN or UN Specialized Agency official who is traveling abroad to carry out official duties. Furthermore, it is an official travel document recognized by every UN Member State. The fact that some migration officers might not be aware of its existence does not in any manner diminish its value or validity. Although it is worth noting that traveling with the personal one in addition to the LP is advised, as a personal experience after visiting +30 countries, I have never encountered a single migration officer who rejected the UNLP.

Under the Convention on the Privileges and Immunities of the United Nations, all UN officials enjoy privileges and immunities as long as they are serving with a UN agency, and the UNLP is one of those. Blue UNLP are analogs to a Service/Official passport, while Red ones are analogs to a Diplomatic passport, therefore, the level of privileges are different for each type of passport.

In short, the purpose for the UNLP is to designate the holder as an official of the UN System it does not express a country-specific affiliation. It does have privileges and immunities (as described above), it also provides time-saving benefits as any UNLP grants you the right to use diplomatic lines at the migration area of any UN Member State and when requiring a visa (for UN related affairs) these will be provided free of charge and in an expedited manner.

When not traveling, the UN issued ID Card is the document that designates its holder as a UN official with the privileges and immunities that it entails and according to the Convention.

As alluded to in Calchas’s answer, the usefulness of a UN laissez-passer (LP) depends entirely on the country to which its holder is traveling. From (indirect) personal experience, I can report that practice varies widely.

• Jordan requires UN employees traveling on official business to have a visa placed in the LP, not in the national passport.

• By contrast, the United States does not generally place visas in LPs, preferring a national passport, although the Foreign Affairs Manual allows it.

• South Africa exempts UN employees from the visa requirement if they present their LP, even if they are traveling for personal reasons. The entry is recorded in the LP rather than the national passport.

• Some countries, for example the United Kingdom, offer a visa exemptions for holders of a UNLP only when the traveler is on official business (“allows the holder to travel to the United Kingdom on official business without a national passport or entry clearance. However, when holders travel to the UK for any other reason, they should use their national passports (and visas will be required by visa nationals)”).

In the United States, at least, diplomatic and semi-diplomatic privileges and immunities are granted based on a person’s accreditation to the Department of State and on their rank and function in the UN system, not on the possession of any particular document. See, for example, 22 USC 288d, Privileges, exemptions, and immunities of officers, employees, and their families; waiver.

As a consequence, for example, someone who holds a UNLP but travels to the US for personal reasons using a B visa or the visa waiver program derives no privileges or immunities from the LP. Other countries, of course, may choose to grant privileges or immunities to similarly situated people.

As noted by Zach Lipton in a comment, the LP may be useful for UN employees traveling to a country that would not otherwise grant admission because of the employee’s nationality. This does not apply in the US, however, because the US has developed other mechanisms to deal with this issue, among which is the C-2 visa.

Is it for proving UN affiliation within a country?

It is certainly one way to do that, although UN employees generally also have an employee identification card.

Without any particular diplomatic privileges is this useful outside of war zones, refugee camps, uranium-enrichment plants, etc?

In many countries, the document does come with diplomatic privileges, but even in those in which it does not, it does serve in some cases to qualify the bearer for a visa exemption. To restate its purpose generally, it serves as a travel document that identifies the bearer as a UN officer or employee. What any country chooses to do with that information is up to that country.

It is used as a travel document in lieu of a passport. It is issued under Article VII of the Convention of the Privileges and Immunities of the United Nations. The Article obliges UN members to recognise the laissez passer as a valid travel document.

Some countries would require a visa for certain passport holders but most countries allow visa free entry for UN laissez-passer holders. Those that do require a visa for laissez-passer holders usually make it much easier for UN staff than people from certain countries (as required by Article VII).

There is also a “red” UN laissez-passer which confers on the holder the same privileges attaching to an accredited diplomat.

I am sure that some people find it easier to use their own national passport from time to time, that is in the nature of any special privilege.