I never have anything in mind when I buy fabric. If I see something I like and it speaks to me I buy it.
I particularly will buy 99% of all Batiks I see and will buy a good amount of solids along the way. I do buy some prints and it all depends on what it is and if it speaks to me or I relize I can put it with some other fabric I already have than I'll buy some.
I will on a whim buy bolts of fabrics if price is so cheap that you just can't leave it there for someone else to grab up.
I am more of a free form improvisional quilter so you never know what I'll buy. I mix all types of cotton fabrics into a piece and in the end it all seems to come out all right.