Being in a fraternity is all about being a member of a social group. In many colleges, especially in the US, the population density is often high enough that you're effectively alone and it causes a lot of students to clam up almost instantly. Being in a frat helps immerse you in the college culture and ensures you maintain some level of social interaction with your peers outside the classroom.
But, it is important to remember that different frats are designed for different social groups. If you are not a devoted athlete, joining an athletic frat is a silly move - you won't have much in common with the people you're forced to hang out with. Additionally, keep in mind that many fraternities have various hazing rituals that can vary wildly in the humuliation/discomfort/bodily harm they cause. If a frat wants you to do something that's utterly stupid, don't join. In spite of what some people might say, the hazing ritual is an important part of the fraternity. It gives the members a common bond that allows for easier interaction. I liken this to my experience in the military. Anbody who was in the army has some shared miserable experience that I can relate to, which means I can almost instantly get along with current or former members of the military based completely off this somewhat shared experience.
Fraternities can be a good experience if you take care in choosing one. I'd go so far as to say it ought to be a required part of the cirriculum - it's that important. Most colleges have a day or two a year set aside where the frats go on their recruiting drive. Go to one and just start asking questions - you'll find out in a hurry if a frat is right for you. At larger universities, the selection can be absolutely huge. Hell, even the community college where I started my schooling had more than a dozen, ranging from fraternities for academic excellence to the standard male/female social orginazations.