I do both but with today's prices I'm more restrained
I usually buy for a project. However, I love MICE so I was at a LQS near my sister and saw some adorable mouse fabric. I bought 3 yards on the spot with no plan, but it is there in my stash waiting for inspiration.
I sometimes collect for a special quilt that isn't fully designed yet. I am currently working on rocks and wood for a quilt that is still not clear on my design board (or my brain whichever) but I know I'll need these fabrics.
I find some of the other answers are interesting!!!