Is Bennett C1 a must?

Yes, he can buff 468 ATK easily. At level 90 + talent level 13 (C5), he buff 139% or 1100 ATK with a 5 star weapon. C1 is kinda a must. Everything above that is extra and C6 is a nono unless you plan to be a diluc main till the end of time.

What does C2 Bennett do?

C2: Impasse Conqueror is another powerful constellation level that will increase Bennett’s Energy Recharge when his HP falls below 70%. This skill is useful to recharge his burst faster when Bennett is running on low health.