I like using flowable resins in the following manner:
After matrixing a Class-II cavity prep, I flow in some flowable to seal up where the matrix band meets up with tooth structure, cure it, then fill up the rest of the cavity with regular-consistency resin (I use Tetric Ceram). This ensures that I won't leave voids in the interproximal parts of the restoration.
The clinical instructors where I went to school also prefer using something like Heliomolar flow for small Class-I cavities.
There are many other applications for flowable resins, but those are the two instances where I use them the most.
HTH!