All characters have something they have to 'watch' that gets fatigued in long fights. Healers and casters have mana. Bards, melee, and tanks need TP (but also use mana). Casters and healers can use TP to dash too.

It's basically there so you are forced to use some sort of strategy/reservation. Tanks need to watch their TP. Melee need to make sure they have enough for rotations. Bards can't overuse their AoE attacks or they'll be completely out of TP and have to wait for more to regenerate. Many special abilities cost TP. There are combos (or latent effects) that trigger when certain abilities are used, including 'free' abilities (costing no TP when they normally would) or an insta-crit.

It has its place like mana does for casters.