My only experience is with Garmin, but the other thing the computer sync is great for, is uploading custom workouts.
I'm not sure what workouts come installed on the Polar. There used to be a series of workouts pre-installed on the Garmins, but the new ones don't have any at all; you have to choose which ones to upload from the desktop software. In any case, they weren't always the workouts I wanted to do. I had to write my own ladder intervals workout, and a program to pace myself for the first 3.5K of a 5K race, for instance.
On the Garmins it is possible to create and edit workouts right on the watch, but it's a WHOLE lot easier when you can see the whole thing on the computer screen, rather than just seeing one step at a time on the watch's little screen.
So again - I can't speak to what the Polar models do or don't do, but being able to customize workouts is another reason to be able to sync the watch to your computer.
Speed comes from what you put behind you. - Judi Ketteler