Exactly what it says - It filters out any small pieces of junk from the gasoline that could screw-up an injector or another system in the engine. But, unless you get your gas out of a rusty barrel in the barn, commercial gasoline is quite clean now days.

It filters the fuel on its way to the engine from the fuel tank

Any bits of dirt / stuff from the tank should be removed by the filter, just nice clean fuel gets through to the engine

Reality on that is its merely a paper filter inside a metal can that can hold fuel under pressure. It can only stop the large particulates. The fine stuff still gets through. they used scintered bronze in the old days for the fine stuff. it was like a thimble at the carburetor inlet.

Thats why they do injector cleaning at tune ups with a caustic solution that could melt plastic. There is no cleaner you can pour in thats truly effective. Injectors also contain a screen to stop the bits like tiny rocks that make it through.

"depositless" injectors require replacement when they clog up.

it just takes a lot of miles.

It filters out the pee that people put in the tank while you are asleep.

That's a real tough one.