This separation is handy if you want to compile multiple versions, for example, one with debugging and one without. There are several ways to do this. If you've already checked out the sources using your own account, this option will allow you to use the source tree you already have. Even if there are no changes, the fetch phase takes several minutes. If all you are doing is verifying that a local change compiles, getting any other changes is probably unnecessary.
Some components offer options for configuration.
The Build system will automatically do the configuration for you based upon the options you provide.