[Johan Hiemstra]

It won't change anything you will tell it to change. The custom icon feature is optional, you can ignore it.

I was under the impression that I install this thing, and it will take care of the rest

That's correct. Just follow the installation instructions above. If you don't use the Template Modification System (TMS) you need to make quite a few template modifications manually.

Your site will load faster, and it can reduce server loads noticeably. The actually difference depends on how much visitors your site receives however. In particular the forum home and forum display pages will load faster and that's important for search engine rankings as well. Since vBulletin is pretty slow by default this is one of several things you can do to speed up the forums and I'll take anything that makes it a more smooth experience for my visitors.