It was initially a helpful construct to suggest that:
* Your potential place of work was not stuffy. That the applicant could seek relief from their horrible bank job. (This signal is now false given that boring bank jobs say "rockstar." :D )
* The applicant will be respected and treated well (like a rockstar), not a codemonkey.
Nowadays, "rockstar" implies the applicant must be all-skilled, all-talented, with no burden on the potential place of work to live up to anything suggested by use of the word :)