Why feed the raccoons?

You're looking at it from a practical perspective and I think the perspective of two Beale ladies was much more romantic, sentimental and nostalgic. Added to that, they had relatively little social interaction (some but not that much) and seem to have openly "welcomed" many into their home, even stray cats and raccoons. Also, it breaks the day up for them - it's something to occupy themselves with if they have to feed the animals. It becomes another topic of conversation (and self-entertainment) for them.

omg so crazy! The ladies are crazy!! Dont they realize the racoons destroy the house!! ahhh

Crazy old ladies always feed stray animals.

Because they're mental patients, notice how the daughter was mimicking the dance moves of the patient from One Flew Over The Cuckoos Nest. And the gene was passed on to the daughter. The house is in decrepit condition on the outside, the coons have gotten inside, probably from a hole near the roof where the wood has rotted from not being painted, the coons have then gotten inside the walls and the man-sized holes in the plaster facilitate easy movement around the house.