Aside from the benefits of not having to pay for it,
A lot of closed source freeware has adware and spyware built into it, and you can't tell unless it's obvious. With open source, there is accountability because anyone can look at the code and see what it is doing. If you are including this type of thing or doing something malicious, it would be stupid to make it open source unless you are up front about what the program is doing.
Have you ever used a commercial program and found things you wish that program could do or wouldn't do? Usually commercial software developers don't give a crap about your ideas. But in the open source world, it's all about everyone working together to make a project the best it possibly can be. With thousands of open source programmers out there wanting to help out the community, things get done faster and you're not limited to one or two viewpoints of how things should be done.
There are a lot of benefits to open source. But I'm not one of those guys that thinks that open source is the right way all of the time, and that proprietary software is evil.