In my case, the answer is both.
For some, like me, feet are naturally attractive if they look a certain way. For me to describe that "certain way" would be like you trying to describe why someones face is attractive to you.
Tickling is also fun. Describing why it is fun is a bit complex, but it doesn't have to involve the feet for it to be fun. Tickling and feet can be entertaining separately, but are wonderful when they are combined!
Since this is the Tickling Media Forum, I would guess that most of the people here are "into tickling and like feet as an extension" vs. being "into feet and tickling is an extension on that". Many people who like feet like them for reasons other than tickling. There are other foot fethishes that I'm not into, such as toe sucking, dirty feet, feet with socks, peeing on feet, feet with dangling shoes, and foot torture (with candle wax and sharp objects, for example). It's unlikely that they would find their way here to the TMF.