Should I review papers for a nameless conference? I received an invitation to review papers in a conference that is not very famous. However, the conference has a web site, and it has been held less than 5 times. Should I review papers for this conference? What is a judging criteria for deciding whether or not do the paper review for conferences?
Does it help you?

A good criteria are the people organizing the conference. Have you heard any of their names? Do you want them to know your name, too?

Also look at the papers published the previous years. Is there anything you like? Because that is probably the good papers you get to review (there will always be much worse submissions additionally that do not get accepted).
If you don't see anything interesting for you &amp; in your expertise, then don't review.

Beware of 'pay to publish' conferences (is the publisher on Beall's list of predatory publishers?). There are companies that live of publishing article no matter what quality. To give at least the impression of being legit, they need names on their program commitee - and they reach out to low-ranked people and invite them. They want to abuse your name!

Don't get me wrong, there are really good small conferences (in particular some with 20+ years of history). But usually they have A) top names on the board, B) very interesting articles and C) no shady publisher, but e.g. Springer, ACM, IEEE, and some just put the papers on the website, or host them at a university library.