When hacking away at a shell or writing a quick on-off script that will only do a handful of things and only once, knowing how fast each part works isn't that big of a deal.  When you're writing something that will get reused over and over, especially if it's something that's working against a large collection (such as, for example, running a WMI query against every computer in your domain), knowing how quickly different commands run could be quite important, especially if you're looking to improve execution speed.

To test execution speed, I use Measure-Command.