As a recent introduction to version control explains, a tag is a way to record a snapshot of the state of some files at a moment in time. When I ask clients why they want to tag their source code, one reason is always "so we can reproduce a build later on". And we generally nod our heads and move on.
The funny thing is that I've never actually seen a tag used for that purpose. Tags do get used to decide whether a particular bug fix was in a certain build, or as references for released builds. But as a developer if I need an earlier build, it's probably to debug a customer problem, which means that the software was already released. Since my build process will have carefully preserved any build artifacts such as debug symbol files, I already have the files that I need.
The other observation is that build systems are designed to be able to use different branches. So if I really needed to recreate a build, I could always branch from the tag and then build using the head of the new branch. Of course, this assumes that nothing else in the environment has changed.
In summary, tags are necessary, but maybe it's not for the reasons that we usually assume.