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!
Yes, you can use the revision number to uniquely identify a snapshot in time. 

But... where do you record which revnum matches which release? Will you keep a spreadsheet that says "release 4.2.1" is Revnum 18345? Not recommended.

However, creating a Tag 'branch' does exactly this without the need for spreadsheets or external tooling - if you remember a branch in SVN is just a copy of the repository state (like a symlink) then you realise that a Tag is not much more than a pointer to the revision you want to remember. You get all the benefit of a snapshot in time, but with the advantage of being able to name it something meaningful. 

You also get other benefits, as its a branch, you can make patch fixes to it. Code made to trunk can be merged into it when necessary. You can create a new branch from that branch so you can make a forked product if you need to. You can switch your working copy to the Tagged release with a single command. All of this can also be done by recording the revnum, but its much easier to use a Tag, and as it doesn't cost anything you might as well go with the easy option.