Quick scan is slightly faster than the Intense scan by limiting the number of TCP ports scanned to only the top 100 most common TCP ports and by using a more aggressive timing template. This scan, also known as quick scan plus also performs OS detection along with the version detection (that was explained a few pages ago). The best example of usage would be mainly for the OS detection itself, as it's the fastest way to retrieve OS and version info in the shortest amount of time, considering the other scans (Intense and Comprehensive scan) take more time.