Trawler

Most boats fishing "trawlers" style have a mast on the stern. What is the masta Is it for the deployment of a sort of veil, or is it intended to support a kind of net or rigging for anglinga I'm curious about the historical and current uses of such a mast.

Their main pourposes used to be to carry a small sail boat to keep his nose to the wind while her work is stopped. Thus, many other uses have been added over the years, he kept his work even if it is not necessary for the sail.

Well, the boat would not look right without a mast, but there is a practical purpose: to support the boom that is used to launch and recover the boat. See this example:
http://oya.com/brokerage/46bonneidee.htm ...

It has many uses. Derek or used as a crane to lift heavy objects, dinghy, etc.
In addition, used to increase motor aa veil.
Used with a sail to help keep the ship rolling in waves.
Used to keep the bow into the wind in heavy weather and / or anchor.
Probably more features that I do not know.

This is a good place to mount radar, spreader lights to light the upper deck and anchor light, and other things like a weather station. It usually has a boom to lift the boats, etc. Some also agreed to stabilize a veil, to reduce body roll in progress, but usually they do not work too well. You can also deck of a sail as a power back to the fold, but this is usually not very effective, because of the size of the mast and rigging vs. vessel size. Another use is to equip a small sail to stop "hunting" back and forth at anchor.