This is more of a "Whether to?" question than a "How to"?

What exactly is the "Product GUID" used for?

I have 4 releases of my installer:

1. The "main product".
2. The "evaluation copy" (as 1 but with some commands time-limited to 30 days).
3. The "major upgrade product" (as 1 but only installs if a legal, non-evaluation earlier version is located)
4. An alpha/beta test version - for testers only (and which drops dead completely at a future date).

The single install script which manages the minor differences in the installation procedure for each is great. But I am wondering whether to give each one a separate "Product GUID" and I can only judge that if I know what it does!

Dave