The relevant portion of the monk's Martial Arts feature states:
When you use the Attack action with an unarmed strike or a monk weapon on your turn, you can make one unarmed strike as a bonus action.
In contrast, the Ki-Fueled Attack optional class feature for monks states (TCoE, p. 48):
If you spend 1 ki point or more as part of your action on your turn, you can make one attack with an unarmed strike or a monk weapon as a bonus action before the end of the turn.
The main difference is that actions which don't make attacks but do cost ki trigger Ki-Fueled Attack. For instance, if you are a Way of the Four Elements monk and you use your action to cast a spell, you can still make an attack as a bonus action with Ki-Fueled Attack.
Ki-Fueled Attack also allows you to make an attack with a monk weapon (which could be a ranged attack), so a Way of the Kensei monk who spends a ki point as part of their Attack action could make a bonus-action attack with their ranged kensei weapon.
Ki-Fueled Attack was added to allow subclasses to make a bonus-action attack in situations where they didn't qualify for the bonus-action unarmed strike from Martial Arts.
The version in Tasha's Cauldron of Everything also allows monks to upgrade an unarmed strike to a weapon attack, if they qualify for both (unlike the version in UA: Class Feature Variants, which only allowed an unarmed strike).