As stated by tons of people, ledge-cancelled Sing is good for catching careless or overly-aggressive opponents that try to pressure you when you're on the ledge. I've also found Sing to be useful for catching opponents who are being too predictable or who missed a really laggy move. Usually though, if an opponent misses a laggy move or is in a really predictable pattern and they're at around 40-60%, I'm going for the Rest 9 times out of 10.
As for Rollout, it seems like quite a few players have no idea how to react to it. While it seems like a move that can easily be shielded or jumped over, shielding the move all the way is a HUGE mistake. If you're able to hit your opponent's shield with it 3 times, it can actually break shields pretty easily, especially if they started shielding early and don't drop their shield between hits. However, most people drop their shield after the second hit, which is pretty much a free 12-ish% for us. It's important to iterate that this only really works on bad, careless, or unknowing players as Rollout can be spot-dodged, but for some reason, it works more than you would think. As other people have stated, Rollout is also good to punish missed techs. I also find some use out of it against players who aren't using a character with a projectile and who aren't really approaching. Last, I think it would be a decent move against someone spamming Counter (with Marth/Lucina, Ike, Shulk, Greninja, Little Mac, etc.), as you could roll towards them to bait the Counter, roll away to avoid hitting them (and, in turn, getting hit), and roll back into them to punish. I haven't really used it for that at all, but it seems as if it could be useful for that.