What can the High Capacity Fuel Tanks upgrade be used for in SC2?
There is no build High Capacity Fuel Tanks is an essential part of. It is indeed almost never researched, because Tech Lab time is much better put to other upgrades.
Players who specialize in heavy drop play, like TY, do sometimes get Fuel Tanks in late game, but then again, it is hard to judge whether Fuel Tanks are good, but simply least good and hence researched last, or players research them just because they can, being in a more or less comfortable late game.