You don't need one, but you may want one. I use ADW.Launcher for the following reasons:

Icons can be renamed (and the labels can be removed, if that is your thing). It's handy because the icon labels are truncated if they are too long.

The Kobo icons at the bottom of the launcher can be changed. I changed mine to Cool Reader, ES File Explorer, Kobo, and Kindle since that gives me access to all of my ebooks.

You can change the number of screens the launcher uses. For example: my main screen is dedicated to my favourite apps while a second screen is for semi-private stuff like calendar, contacts, and email. The other three screens simply don't exist.

The "grid size" can be changed. I set it up to display 4 icons across and 5 icons vertically since the larger spacing makes it look less cluttered.

The launcher can also rotate to landscape mode, which makes it much less disorienting when switching from an app that is being used in landscape mode (example: Adobe Reader) to the launcher.

Those are just a few examples. I'm sure that I'm missing a few that I actually use and a lot that other people use. There are also much more powerful launchers out there with a dizzying array of additional features.