Why does Willy keep planting seeds where they've never grown before? Why does Willy think Biff will be impressed with his funeral? Why does Ben say that Biff will call Willy a fool?

Willy does not wish to give up on his hopes for a future and some kind of memory of himself or his existence after death. This is why he plants the seeds. He hopes that his funeral will be well attended, and that this will impress Biff. Biff sees that there are much more important qualities than popularity, which is why he will see this final act as foolish.