People argue to find out (more information, more clarification, etc.).
Arguing is (or can be) healthy as it allows people to learn about what they are arguing about. By arguing, it forces the problems to be addressed.. whereas being humble, meek, or quiet would not necessarily educate the arguer or debater. Even if one person agrees with the other, arguing in a devils advocate manner can force out knowledge when it otherwise wouldn't have been forced out by being quiet or agreeing.
Especially people argue to find out on the internet, where it is easy and cheap to initiate conversation. There is not as much risk, such as getting in a real offline pillow fight or other violent act. There is less risk of time being wasted, since the internet is quick. However, people will still argue to find out offline where it is easy enough to.
Somewhat OffTopic but interesting: it could be that in a healthy marriage, arguing to find out is healthier than bottling it up (at times). Arguing to find out hopefully does not lead to more serious issues such as divorce.
SucksPages also can be very healthy, as since EverythingSucks we know that WhatSucksLess is what is most important.
See also: AgreeToDisagree, AnalRetentive, NoKeening, HowToAskQuestionsTheSmartWay