Thank you very much for your replys!
Mixing in mono doesn't really mean your final product will be a mono file but listening or monitoring in mono can help you figure out some issues in the mix that you may not hear in a stereo mix.
This is a good article that explains it. http://www.homestudiocorner.com/mixing-in-mono/
David, I think with this article I'm answering WHY
I think that what can I do with this technique is just EQ and gain staging, and I doing more. That was my question.
And yes David, I think you are right, it is just checking, no mixing in mono.
Thank you again for your time guys!