I used GUIDs because they allow easier tracking across databases and external servers.  For example I can tag export or import files with a FileUID and carry that FileUID across servers or databases without fear (just anxiety 🙂 ) of conflict.

They can also be usefull for editing records offline and re-syncing back to the main server.

They are also nice for restoring data in a table (like an accidental delete query) and not worrying about keys getting regenerated of conflicting with existing data.

I add a GUID to all my tables, not always because I have an immediate need for them but for later use.

Btw - I never use a GUID for a clustered index but if my dataset is mostly static or very small then maybe I would and cringe a little.