I had a uncle who was one.
He often had a backup place to live in bad weather ( a nasty daily/weekly rental) but not always in summer.
He had mental / emotional "issues". He was gassed and bayoneted in WW1 and was never quite right again. He got some money from the government, so he didn't beg or take money - but he did (as he put it) "camp out".
My observations working in downtown D-town are that the bums look like:
30% mental cases (building talkers, mumblers, and the semi-catatonic...)
65% self-inflicted (burnt-out drunks & druggies)
5% lazy scammers (not necessarily that they're making $60K a year, just that they have "reduced expectations" and are comfortable with them.
This is the Pub. Leave common sense at the door.