Well, here’s three good examples.

Planning:

It’s far faster (and moreover easier) to mark points in Google Maps than it is to do the same on a Garmin GPS. This is because you’ve got a full computer keyboard, monitor and mouse at your disposal.

The largest advantage with Google Maps is that you can literally take your mouse and drop it anywhere you want on the map, then send that as a location directly to the GPS without worrying about latitude/longitude coordinates whatsoever (it’s auto-calculated). Yes, you can do this on Garmin GPS devices as well but you don’t get satellite view, street views or terrain views. Using Google is much easier.

Sending to other Garmin GPS owners:

Basically put, if you don’t know the street address of the location you want to send to another Garmin GPS owner, you’re screwed (unless you know how to make a POI database which is a huge pain – I know this from personal experience).

With Google Maps, you mark a location, link it, and IM or e-mail it to your friend/relative/whatever that has a Garmin GPS. Then they can load in waypoints the same way you do. No fuss, no muss.

Collaborating:

There is no way to have a collaborative map data set between multiple users via the internet – except with Google Maps.

There is the ability to “share” a map with Google Maps where people of your choosing can add/edit/remove locations. For people that band together for road trips (such as motorcyclists often do), having a data set that can be *easily* worked on between users is a godsend. Everyone will get the same data to send to their Garmin GPS receivers.