Masking | General tips

[frmgsft]

Hello,

I have no experience with using masks and I wanted to ask you guys for some tips for a newbie.

First of all, what are benefits of using masks in general? Is it a higher quality reconstruction, decreased computing times, etc?
Are there any scenarios that masking is better suited for, or any that masks should not be used for?
Would there be a difference between using masks versus images with deleted background?

Please advice.
Thank you.