Backpressure on gasoline engines: is it important?

Hello everyone,

It doesn't really fit in the TD forum, but I haven't been able to find anyone who could explain the connection to me so far.
The topic of back pressure/counter pressure in the exhaust system has already been extensively discussed for diesel engines. I'm now interested in how these effects manifest in gasoline engines.
In this specific case, it's an old 2.9L 6-cylinder engine from 1986. Ford engine with electronic fuel injection (EEC-IV) and a custom-built exhaust system.

1. What is the purpose of back pressure?
2. Low back pressure => high consumption?
3. Effects on speed and torque?

I hope you've driven a gasoline-powered car before.

Thank you very much and best regards.
Stefan.