Upvote:3
In order for the Allies, specifically in this case England, to create a naval blockade against the Axis--Germany in the north--it was practical for England to cover as much area as possible without physically being present with warships.
During WWII Germany had solidified it's reach to the Netherlands, Poland, northern France, Belgium, Denmark, and Norway. All of these areas were potential ports for both German imports and exports. With that, it is necessary to cut off any potential traffic through that gigantic area of sea.
Of course, as it was implied above, England had its own interest to protect her own ports and ships against German submarines who were incredibly skilled and populous especially in the Atlantic seaboard and the areas identified above.
Nonetheless, this question is more complex than the answer above. There are many details involved. Rather, I have given a general answer.
Upvote:8
The initial British/French mining in the channel was as an anti-submarine barrage. Later anti-invasion fields were laid.
Later in the war offensive fields were laid on the other side of the channel.
Details of RN mine laying in WW2 may be found here