• It’s interesting to see how other people write code, from their specific syntactic style to the level of commenting, overall design and code layout, common idioms etc. You’ll probably learn something – either good things that you can copy, or bad things that are tough to read and warn you off doing the same.
• You may find the code frustrating and difficult to figure out, not only initially but no matter who long you work through it, though if that’s the case then it’s probably poor code. Hopefully that will influence how you write your own code, for the better. 
• If it is a product that you use regularly, you may gain insights into what it’s really doing and how, that empower you to use it more effectively. You may even be able to modify it to make some improvement that matters to you.
• Once you’ve got past the initial difficulty and started to understand things, you acquire a sense of power. No longer are these complicated pieces of software opaque and mysterious. They can be broken down, understood and messed with. A whole world of possibilities opens up and you no longer need to be afraid. 

I’ve recently been reading the Ruby source code of Innate – the new underpinnings of next generation Ramaze, a Ruby web app framework that I particularly enjoy using. Innate is built on top of Rack, so I’ve been reading that too. The mystery is ebbing away and it’s making me more powerful. Stand back! 🙂