Question: In my company, we have had a policy of tagging every release. Someone new joined, and he suggested that instead of formally using a Tag, we could just leave a comment for the release build when it is checked in. I like using a Tag, but, obviously, we can also get to the source code for a build also by looking for the comment. The only advantage I see is that because our product spans multiple technologies, we can group the source code for both in the same directory with a folder for each. **Is there some other advantage of tagging releases that I'm missing?** BTW, we are using SVN.

Answer: 
A tag is precisely made to be able to get the exact snapshot of the sources at the moment you created it. It is 1000000 times more convenient than looking at the submit comments....especially if your release requiered several commits (or more).

Furthermore, the case where a important bug is dicovered in acceptance or in **production** while the development is ongoing and not stable enough is quite common! 
In that case you have to create a branch. As others mentioned, a tag is beyond the scene a branch in SVN so you are saved in that case but tagging can also save your life with other tools because many tools allow you to create a branch from tag (CVS, TFS and certainly others).
Just try it with comments only!
Another reason to use tags instead of revision numbers to track what you released is it's easier to keep links/references to tags than to revisions of trunk (or branches). References to particular paths at some old revision can break if you reorganize your trunk (or branch) folder layout in the future. Tags don't change (by convention), so they don't have that problem.