Could you vs. Can you?

1) Could you please help me with this?
2) Can you please help me with this?

Which one is correct? I've heard people use the first one a lot but why should you use Past Tense when asking someone to do something?

It's not past tense in that case. It's just a polite way of making a request.

With the modals, past tense does not mean past time necessarily. We use it to create a distance (social) between ourselves and the person we are talking to. This is probably because we don'y have two, or more, forms for the second person pronouns. ;-)