In today's WWE, vignettes promoting the debut of a soon-to-be member of the main roster aren't really necessary. Thanks to NXT, fans are already well acquainted with the company's rookies, and even without it, many of them have extensive backgrounds on the independent circuit anyway.
In previous eras, however, they were almost ubiquitous. Seldom did a WWE wrestler take their first steps in the ring without having to film an elaborate promo delivered directly into the camera, often while standing in the middle of an on-location setting tangentially related to their new gimmick (a beach, a graveyard, a funfair).
Though comically over-the-top, these vignettes more often than not served a purpose, namely introducing an upcoming character to the audience and giving them a general idea as to the type of person they were. In short, they worked - hence WWE kept using them as a means of promotion.
That said, there are a small number of examples of elaborate vignettes which didn't quite succeed in setting the featured wrestler up for a long and successful wrestling career. In fact, here are a bunch which barely made it into the ring...