i'm a big fan of pre-cuts for a lot of reasons.
For variety - a little bit of a lot of different fabrics. I love to make multi-coloured or 'scrap' quilts, so they suit my personal style.
Then, convenience: it's great that someone else did the cutting for me. I've only very very rarely found mis-cut squares (often, unfortunately, during fabric swaps).
Then, to keep up with colours and trends: not having quilt fabric shops anywhere near me, I enjoy receiving sets of squares or fat quarters and have in the past been a subscriber to the Cotton Club and to some of KQ's medleys of the month. I've just finished making a multi-coloured '10-minute' block quilt which used 210 4" squares that I'd been accumulating since the start of this century... I loved making it. I also make FQ pillowcases regularly.
They're also fun to play with. They're pretty. A lot of dream-power for a not too big investment. There are lots of cool things you can do with them...
Did I mention that I really love pre-cuts???