All sorts of uses for them.
1) Decoy throw/Bait toss [People rarely think of this one or try it, it works!]: Throw it away from you in an odd direction hopefully causing a distraction. Next reposition!
2) Area denial: Pathways between cliffs, door ways, hall ways, death-boxes, airdrops, down teammate pickups. Toss them to deny your enemy quick/easy access.
3) Defensive throws/Caught off good or out of position: Don't be shy here throw away and run back and get ready for a fight. If you tag them push on in.
4) Reposition throws/Offensive: Enemy hiding behind a rock? No problem toss it at them to force them to move proceed with an aggressive push or gun fire.
5) Offensive/Aggressive: Self explanatory.
6) Finish off downed players: Busy fighting others? Don't have time to finish that guy off making sure he isn't picked back up? Yeah toss a few at him.
7) Teammates on their toes?: Playing with randoms? Want to know if they're at least aware and care about the match? Throw a few tossables randomly! If they react they are in it to win it! They will spin in circles looking. They will run to you. They will chat where are they? Sure you could shoot your gun, but it doesn't make the same impact a tossable does. Normally it's best to do this test at the early stages of the game when you don't plan on carrying them or have extra.
8) Flame grenade thingy is basically useless, not worth the bag space. Yes you can use it but why carry it when the other two are far superior and just as plentiful? IT NEEDS A BUFF! Either a larger cover area, more damage or an increase in carry to 4.
What not to do or bad examples that I read here so far
1) @ The guy using them to take out caustic traps bad idea. You have limited carry space for tossables you should use your gun to take out traps. The only reason I could ever see using a tossable on a trap is if it's an odd angle and you're bouncing grenade(bunkers.) Even better method is avoid his traps all together. Run past fast or stay far.
2) Be careful of random objects like trees they will bounce them back at you or attach if it's an arcstar.
3) Don't carry more than 4 total ever; Ideally 2 arcstars and 2 grenades. Small backpack I only carry 2. No backpack probably none. Your load-out can change this as well. You don't need a lot of sniper ammo or shotgun ammo currently allowing for more bag space. This is being changed for shotguns forcing you to carry more stacks of it. Sadly I don't believe this is being addressed for the longbow or other snipers, wingman even you can get away with 1 stack.
4) I'm sure I forgot something and could easily write a book on all this, but who has time for that.