ok, one of these probably happened to you:
1)you're in a tight space with redstone things.want to place a piston facing a block.You'd have to break a block, place the piston and put the block back
2)you're swimming in an ocean, its very deep and you need to craft something so you pillar from the bottom to the surface to have a platform
3)you want to place upside down stairs/slabs but you need to place a block above where you wanna place it
4)you want to place a door but facing outwards, you'd have to go to the other side and place it.
An alternate placing key!
this key (bound to left control, for example) when held would allow you to place it and an oposite/alternate way, like placing upside down stairs/slabs, place pistons/repeaters/dispensers facing away from you and even placing on water!
I really hope this becomes a really handful mod or maybe even reach the vanilla someday!