You don't "need" backpressure per se...

You will get the most power with the highest exhaust velocity... backpressure slows it down... what you really want are the right diameter pipes for the size of your engine/power output.

Backpressure is often used as a "fix" for a poor tuning job...

You do want to eliminate power-robbing negative scavaging effect... but a good pipe will do that with minimal backpressure.