Why they're an all-around great idea.
I've had five roommates throughout my life:
J is still my number one best friend despite living in separate cities five hours away from each other. Each person brought their own benefits to the table and helped me out in different ways.
Roommates greatly reduce housing costs. For homeowners, a single roommate can bring in an extra $500-1000 per month. For apartment renters, I've seen 1B1B vs. 2B2B apartments going for $1600 vs. $2000, respectively, saving $600/person.
It's nice to be with someone at home even if you don't talk to each other.
Mostly for homeowners, having a roommate helps to make sure any issues that arise (power outage, leak, burglary) are found and dealt with quickly. I travel about a weekend a month while my current roommate rarely does, so it allows me peace-of-mind when I'm out that my house is fine.