It's the key to maintaining your sanity
When you don't have bosses or coworkers hanging around watching you, it's easy to spend half the day watching videos on YouTube, or just mindlessly surfing the web and hanging around in forums. You look up at the clock and realize it's almost noon and you've accomplished nothing. Gonna have to work until 7 or 8PM to make up for it!
This starts a long slide down the slippery slope that erodes the barriers between your "work life" and "home life." It's not until you have left office culture that you realize how useful it is to A) have clearly-defined working times, and B) have a whole entire separate location where work gets done.
When you set yourself a hard and fast deadline for ending the work day, it forces you to tighten up your work practices earlier in the day. You can look ahead and tell that "If I want to get that project done by 6PM, I had better start working on it now!"
Having a distinct end time also helps solve the "blahs." We have all had days in the office where we were just running out the clock until we could go home. That doesn't happen when you are your own boss - if you aren't working, you're not getting paid! But strangely, being able to look forward to a specific time that signals the end of the day can give you hope. Like a cart horse with the barn in sight, you may find yourself picking up the pace as the pre-determined end of the day approaches.
Most of all, be sure to set yourself a sane work schedule. It's easy to get over-committed as a freelancer. And it's a good problem to have! But study after study has shown that the less time you spend working, the more work you get done. You do more in a 40-hour work week than you do in a 50 or 60-hour work week, counter-intuitive though that might seem.
No matter what, be sure to draw your boundaries between "work" and "home," and defend them with your life. It may be hard to resist the urge to put in a little extra time on a Saturday afternoon, but you'll be doing yourself a favor if you use that time to relax instead.