Having ADB and fastboot available outside of the other two main downloads, first of all, saves space. Android SDK takes up 292 MB of space.

Thanks to the work of two Googlers, there are now direct links to the approximately 3.5MB platform-tools folder that includes adb, fastboot, and several other utilities.

You can get the platform tools for Windows, Linux, or Mac direct from Google's servers. You get a ZIP file containing the platform tools folder with ADB, fastboot, and a few other tools.