Work with any code, easily
Languages and libraries aren't always well-documented; legacy code isn't always well-commented. Understand how APIs, components, frameworks, and all the critical code you use really works by decompiling and seeing inside.
Having the source code available means you're no longer blocked by poor or missing documentation. So you can see how code runs, avoid bugs and develop more easily with 3rd party technologies.
Replace poor or missing documentation by seeing how the code works in context.
Find unexposed and undocumented functionality and get more out of the APIs and technologies you're using.
Drill into how .NET languages and frameworks are implemented, and learn how to use them.
A detailed technical study using .NET Reflector to see how the new async implementations work.