Tnt is useful for it's efficiency. Gunpowder can be automatically farmed through relatively simple means, while sand (for instance) is not so easily automated.

Tnt is useful for it's efficiency. Gunpowder can be automatically farmed through relatively simple means, while sand (for instance) is not so easily automated. With a few well placed blocks of tnt, entire inventories full of sand can be collected in moments compared to digging it block by block. Minecarts and their tracks are useful for automation of resource transmission- while faster modes of player transportation exist, it's difficult to transport thousands of items by hand- with a line of tracks and some simple automation, this becomes trivial.