what the perpus of the white fence on the loft roof

I believe its to try and stop the pigeons from landing and hanging out on the roof of the loft.

I thought so.

Yep, and to get them trapped quicker....you will also see some roofs that have very steep pitches so the birds can't perch.

Also helps in preventing hawk attacks. If your birds are stubborn and still want to stand on the roof behind the fence, a hawk will less likely attack because they like clean getaways after attacking. So if a fence is in the way, it'll take more work to get around it (especially if the fence isn't just on one side of the roof), and it will either slow the hawk down or discourage it completely