I use them under just all all of my composite restorations as a "liner". The way I place mine is how I learned in a CE course from Dr. Buddy Mopper (He owns Cosmosdent). After etching and apply my bonding agent and light curing the bonding agent, I'll place a smnall amout of flowable into the prep and then before curing the flowable, I'll put my 1st increment of my "regular" composite and compact/sculpt. This way, the flowable is being squeezed/spread onto/into all apsects of the prep. I wasn't having too uch post restoration sensitivity before I started utilizing this "wet flowable" technique, but nowadays, if a patient comes in with post placement sensitivity and the occlusion is OK, I can almost guarentee that its a going to be and endo and not any deficiency in the entire dentinal tubule/bonding agent/restorative material interface casuing the problem.
I'll also use flowbale as my material of choice in small class V's, and small class I's. It's also great to use to "touch up" areas on temporary crowns made with auto polymerizing resin.