The Gypsy Morph
Being in a compound gave the residents access to things like Pleneten, which those outside the compounds generally could not get.
People lived in them believing they would be safe from infection and exposure. They were able to grow some food, depending on what soil and water they had to work with.
Compounds seemed to be successful until the Once-Men began attacking. They were always of sufficient strength and number to besiege the compounds, turning them into death traps from which the defenders could not escape.
Despite these attacks, and what happened to the people taken as prisoner, people living in compounds still believed them to be the best option. Outside the compounds were Freaks and all sorts of creatures and dangers they did not want to deal with.