I definitely see your point, it's basically just a flashier resume.

Typically web developers, front end, and full stack have a portfolio. Where most of the reasoning behind it is simply influence from social media where people mention it's a good way to stand out.

From the amount of research I've done trying to find portfolios from back end developers it seems like they don't share the ideology that it's a good thing to have.

I think it might boil down to the possibility that they typically don't have an option to include a "personal website" on a lot of their job applications, that's my reasoning for having a portfolio.