Why Find a Roommate?
Everyone is different in terms of what they can/not tolerate in someone with whom they share a roof/bathroom/kitchen/hallway. A roommate search is a good way to figure yourself out, possibly as lower-investment "training wheels" or a refresher course for more important relationships.
If you can't learn to make it work with at least a couple more casual roommate relationships, you're pretty much doomed to wreck things when living with a long-term significant other . . . or doomed to start your next roommate search sooner than you'd probably like.
Living alone can be scary at times, especially in a new place.
It is often safer to have other friendly humans around your home.
A roommate could also be home with your stuff at least part of the time that you're away.
Emotional Support and Safety
A roommate can be nearby and hopefully helpful during a personal crisis.
Not paying all the rent/mortgage and utilities on an entire dwelling is an easy and quick way to save money - for other financial priorities (school/art/hobbies), to afford more of the finer things in life, or to save up for your own place/your own mortgage for the future.
That is, if your roommate search turns up the RIGHT roommate.
You will need to find a roommate that works with your personality and lifestyle, at least long enough to stick with for a good chunk of time. Otherwise, the multiple deposits you'll have to pay for new roommate situations and/or the time you'll spend looking for a new roommate will put a serious dent into the time and money you'd otherwise be saving.
If you find the right roommate some of their friends may become yours and vice versa.
More Space for Less Money
Even though you'll be sharing a lot of it, living spaces with multiple bedrooms also tend to have larger living rooms, kitchens, outdoor areas or balconies, etc.
If you ever feel a little claustrophobic, this can't help but be an improvement over a studio-type situation . . . as long as you get along with your roommate.
Expand Your Mind
A roommate search that exposes you to others that are not exactly like you, your family, or current friends will teach you important life lessons about, you know, diversity and different people and other people's issues and stuff.
Cut Back on Boring Tasks
. . . like those involved with moving to a new city, or moving away from your family for the first time. Moving into an already established household will drastically cut back on the number of moving-related chores. There will already be electricity, probably a sofa, maybe some spoons and forks you can use too, etc.
"Test-driving" Your Social Skills
. . . learning how others react to you up close and personal. When feeling offended by something, is this something most people would find not such a big deal?
Am I totally self-absorbed, or am I more or less "normal?"
What should be tolerated as a "normal" part of living with anyone vs. behaviors that make someone look like an asshat?
As fledgling adults . . . or even adults re-entering a social market, most of us need some practice or reality-testing here at some point. An intelligent roommate search will provide you with just that.