Swarms of Flies?

Can someone please explain the swarms of flies that seem to like to hang out under building balconies and entryways?

I’ve never seen anything like it before, but over and over I’ve noticed that swarms of flies are at the entrances to many buildings here. They’re not annoying and move out of the way as you pass through them, but I just keep noticing it all the time and find it a bit strange.