Question about bare repositories

Hello,

I'm having an understanding problem, rather than a git problem. It came about because I was looking at a repository using gitweb. The repository was bare, and happened to have HEAD pointing at an old branch that hadn't been updated for a long time. I was confused because I wasn't seeing new commits. Once I set HEAD to master in the bare repository, gitweb showed the new commits.

All that led me to this question, which isn't a criticism, but has exposed a hole in my understanding of git.

Why does a bare repository have/need a HEAD?

Andy