It would appear that the colon in git push origin :<branch-to-delete> is used exclusively to delete branches. But such is not the case.

The format for the refspec is*:

<source>:<destination>

This tells Git to push the source branch to the destination branch in remote. So if the source is blank, we get a leading colon. This has the effect of deleting the destination branch. Its like saying “push null pointer to destination”.

*You can learn more about the refspec in its entirety in this Stack Overflow