i'm just starting to use them now and they seem useful. Kind of like rules, your team will play OK without them but they can help give you those tiny edges that are often the difference between winning and losing at the highest level.
Two instances of where i have used them is where a had a backup LT who was a 'pass-blocking' specialist (better than the starter). In obvious passing situations/formations (like 5 WR) he goes in.
Another was the split-backs formation where there is a pitch option and the formation normally uses a (fast) RB and (slower) FB to take a naked pitch. When the FB takes it he goes nowhere, so i substitute another RB for the FB.
The uses are really only limited to the imagination but time ROI is important to me. If my team is so bad that none of the GP stuff will make any difference i concentrate on more important things like obtaining and training better players.