Powder bags would (in a bit of a misnomer) be composed of entirely gunpowder.
They would not do explosive damage at all(except to set off other explosives), but would have twice the "push" on entities as opposed to TNT.
Also, when ignited by anything, it goes off immediately.
Why would we use this? Well....
#Detonators: When you want your explosive set off NOW.(Not for mines though-the gunpowder might just launch you clear of the TNT explosion!)
#More powerful and safer(to operate) cannons:
#Traps: Push your enemies off that cliff!
And why would we not?
#No block or entity damage
#Mines:Again, the gunpowder explosion might blow you clear of the TNT explosion.