Some people using manual tagging as a mechanism to communicate something to CI as you suggest, but others let CI tag successful builds. When I re-did my team's CI a few years ago I chose the latter because it creates less friction for continuous deployment -- every commit which passes the tests automatically becomes a deployable artifact with a monotonically increasing build number which can be tracked across SCM, CI, and deployment.
Heh, you've never worked with a project/product manager, have you? "Yes, ma'am, version 1b6411f has been deployed successfully. Now I just need to edit the pom and bump the version to 2e1080d"
