The origin of the idiomatic phrase to weigh the "pros and cons" of a situation or thing is in the Latin language. The word "pro" in Latin means "for" or "in favor of" something, while the word "contra" means "against" or "not in favor of" something.
To measure the pros and cons of a situation or thing means to evaluate its good and bad qualities. For example, a teacher may evaluate the pros and cons of using games to teach students a new language as opposed to more traditional methods, or a business may have to judge the pros and cons of doing business in a new location before making a decision.
Making a list of pros and cons usually ends in the making of a decision one way or another. A person who is in favor of something can also use this tactic, directly drawing from the Latin definition of terms, to convince another person who is against the same thing. In such a case, the person in favor can gain advantage in the argument by seeming neutral in the presentation and even presenting a counter-argument to his favored position before displaying evidence and developing support for his position by logically refuting the cons.