Why are there "pre" url's for website's and whats the benefit?

I've been doing a lot of research for web development and googling, and I'm not sure how to search this question in google.

I've noticed some bigger and smaller sites have their websites "mywebsite.com" and have all their inner pages such as "mywebsite.com/about" or "mywebsite.com/videos"

Makes perfect sense to me. But then I come across some website that have a section(?) BEFORE the .com, for example "video.mywebsite.com" or "news.mywebsite.com"

I own a couple websites myself and I'm curious to why this is done, and what benefits this brings? Is it better for SEO?

If it is something that I could benefit from, how would I go about doing this if my websites are built in WordPress?