Bennett

4★

PyroSwordSupport

TL;DR

Bennett's Burst drops a circle on the floor. Anyone standing in it hits much harder and anyone below roughly 70% HP gets healed instead until they're topped back up. That's it, that's the character. Use his Burst, park your damage dealer in the circle and enjoy your inflated numbers.

Rotation

Tap Skill (E) once for energy particles to charge your Burst. Don't hold it.
Press Burst (Q) to drop the circle where your team is going to fight.
Swap to your damage dealer and stay inside the circle.
Come back and press Q again when it runs out, about every 15 seconds.

Abilities

Skill (E)	A swing of fire that knocks small enemies around and spits out a pile of energy particles. The particles are the point, the damage is a bonus.
Burst (Q)	Drops the circle, which lasts about 12 seconds. Anyone in it above roughly 70% health gets a big attack bonus. Anyone below that gets healed back up instead. This is the reason you brought him.

What actually matters

The two or three real decisions. Everything else is noise.

Where you put the circle. It doesn't move. Drop it on top of the enemies, not where you happen to be standing when the button comes off cooldown. If the fight moves, your buff move with it.
Energy Recharge. The buff only exists while the burst is up and the burst is expensive. Stack Energy Recharge until you can cast it every rotation without waiting around. Roughly 200% is the comfortable number if he's your only Pyro character.
His attack stat feeds the buff, not his own damage. The bonus your team gets scales off Bennett's base attack, which comes from his weapon and his level. Levelling him up genuinely makes your main damage dealer hit harder, which is not true of most healers.
C1 is a big deal. At Constellation 1, the attack buff applies no matter what percentage health the character is at. Below C1 anyone under about 70% health gets healing instead of the buff, so a carry who just took a hit loses the attack bonus until they're topped back up. It's the one constellation worth chasing on a 4-star.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Energy Recharge, as much of it as you can find. Favonius Sword is the nicest pick because its passive coughs up extra particles on top, but any sword with Energy Recharge on it does the job.
Artifacts	4-piece Noblesse Oblige. Using his burst gives the whole party 20% attack on top of the circle and he's using his burst constantly anyway.
Main stats	Energy Recharge sands, HP goblet, Healing Bonus circlet. Ignore crit entirely, he isn't here to deal damage.

Works well with

Hydro

Cryo

Electro

Honestly he works with anyone holding a sword, claymore or polearm. The elements above are just the reactions his Pyro sets up while he's at it.

Don't bother

Holding E. While it does more damage, it's a waste of time. Tap it.
Building him for damage. Crit rate on Bennett is wasted stats.
Fussing over the exact Energy Recharge number. Get it high, cast the burst, move on.
At C6 he sets your on-field character's normal attacks to Pyro. Great for most people, annoying if that character wanted to apply their own element, so if your numbers get weird after C6 that's why.

Want the full theorycrafting version? Read Bennett on KeqingMains.