Why over-sampling and filtering... and why not?

I wonder if you all might help me understand in layman's terms the OS and filtering question. Obviously, manufacturers added oversampling and filtering to their cd players to correct some perceived flaw in CD playback. What were they trying to correct?

Conversely, there seems to be a small but dedicated movement building players with either no oversampling or no oversampling and no filtering. Obviously, they believe there is some sonic advantage to doing this. What is the benefit in this case, and why did they give up on oversampling and filtering?

In short, what is one gaining and losing in each case?

Thanks all, John