I recently purchased a copy of Ardour, and I have noticed that, but I have found most basic plugins can be found for free.

That's true. In fact, Arch linux even has a handy package (pro-audio) that installs every plugin you'll need at once. Still, some people might prefer to have plugins built-in or it might be too much hassle for them to look for every plugin from outside (it certainly was for me when I was starting out)