i hear constantly about how people have their pistons machined to lower the compression of the engine

but, what are the actual gains of this? just allows for more boost capacity? does it affect day to day running?

also, what sort of compression should i be looking at having it machined to? i have everything ready for my engine to be rebuilt, but wanted to look into compression lowering before i have it all assembled. i have brand new mahle pistons, so it would be these i wud have machined

any advice would be appreciated