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 in subversion is technically the same as a branch. It is only convention that keeps you from modifying a tag after it was created.

Last year I had a situation where we were very happy that we use tags in subversion.  

We had created a release and tagged that release. In parallel to the acceptance tests on the release, normal development continued on the trunk of the repository. 
The problem was that late in the acceptance tests, a critical issue was found that absolutely had to be solved, but the new stuff that was being developed could absolutely not be included in the release, because it was far from being stable enough.  

We resolved that finally by making use of the fact that under the hood a tag is the same as a branch, so we made the change directly on the tag. If we had used check-in comments to 'tag' our releases, that would not have been as easy to do.

Besides that, if your project consists of multiple projects that evolve at a different rate but are always released together, then a subversion tag ensures that you have a correct snapshot of all the projects at the time of the release, even if some of them haven't changed since the last release. If you use check-in comments to mark a release, you must make a check-in on each project, even if nothing has changed.
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.