It also helped to know some things that Bill for all his brains and books didn't know. Jess found he was really useful to him, not a nuisance to be tolerated or set out on the porch like P.T. (7.19)
So, money can't buy everything, and having a formal education doesn't teach you everything either. Jess is glad to discover that even though he feels so inferior to Bill in terms of education and smartness, he still has some knowledge that is valuable. It's important to Jess to be seen as "useful," and sad for us to see him worry that he'll be evaluated as "a nuisance" like a pet dog.