Productivity
If you are doing serious software development you probably have automated builds (via TeamBuild or some other tool) which build all the build configurations you have, run all your unit tests, run code analysis rules, run source analysis rules and more. While writing code you probably want to have all those extra tools and checks turned off because they will drag your productivity down. When you are about to check-in, however, you will need to make sure your solution builds without errors in different build configurations as well as run all tests to make sure you are not about to...