One of my main reasons is the prohibition to discuss transfer listed players on the forums - if you want to find out something about a players true value it helps a lot to look up a similar FA and see how much he is sold for. TPE is useless in most cases...
Another reason is that I rather prefer "burning" my money than funding another team.
Third reason: many FAs come from smaller countries and if you want a NT-player/prospect you will find those often enough among the FAs - so even if I dont look for a specific player I might want to browse the FA transfer list without adding any other filters (some FAs price "skyrockets" in pretty fast so you wont find them with low starting bid settings...