You already have something in common.
Much like when you go to school with someone, work buddies already understand the joys and frustrations that come with the job you have. Whether you're working a job in your desired field, or just working one that will just get you by, your coworkers will understand. Take advantage of this common ground, especially if you work in customer service.
You have someone to complain to.
"Havin' fun, yet?" my best work buddy asks me every time he passes by. While I never offer an answer, it seems a consistent segue into discussions about whatever is pissing us off -- be it work, family, the weather, etc. While some would argue that you can't be friends with someone when all you do is vent, I say those relationships are rare and special. I speak for both myself and my work buddy when I say I leave work in a better mood on days they work alongside me.
There's no pressure.
Unlike with other friendships, where you feel the pressure to always make time to hang out or spend additional energy on that person, most work buddies are content to just spend time together at work. Which is good, seeing as you spend hours on end doing whatever mind-numbing work your boss has in store for that day, and any social skills you have are numbed the second you clock out.
You can be closer if you want.
While a lot of your work buddies are content not to hang out, outside of work, a lot of them would be equally glad to hang out, should you ask. Especially if your shifts end around five o'clock, so that you can go to the nearest bar and indulge in a much needed happy hour, complete with conversations about the ridiculous things you witnessed throughout the workday.
Time goes by faster.
Tyrannical boss? Needy customers? Other coworkers who won't do what is asked of them, leaving you to pick up all the slack? Having someone on your side alleviates a lot of that frustration. Sure, there will still be days that inevitably crawl by, but with friends around those days should be fewer and farther between.
It alleviates summertime loneliness.
I might be the only one, here, but I get lonely as all get out in the summer. Yes, my parents are fantastic, but there are a lot less people to talk to when you're home from college. While not the same, work does help me from turning into a complete hermit crab, and I owe my work friends all the thanks for that fact, alone.