If you run websites that use PHP, you will find that more often than not, you will need to make changes to their appearance, but of course this will affect the actual live sites, if they are running live at the moment.

Also, you may want to launch a site, but need to tweak the template so that it looks fine, and you'll definitely want to troubleshoot issues before they arise, and before the site goes live. If so, you need to install a WAMP stack called XAMPP, and test run your website there.  The beauty of it is, you just need to run XAMPP in a USB flash drive, and it will run right off the bat! You can actually turn your USB stick into a virtual Apache server!

This method works fine too, for putting your blog through a test run. Here is more detailed info on running a WordPress blog in XAMPP, right on your desktop.