Why render designs?

How do you use mockups, specifically high quality mockups (not wireframes), in your design process and why/would you recommend them?

I'm a developer, in the past my product owners would sometimes give requirements in the form of mockups, but even then it generally struck me that it could've been communicated without all the extra work (0 artistic ability w/ a pen and paper sketch would've sufficed).

Last week I was watching some YouTube videos to get some design inspiration for a side/hobby project I'm working on and found myself emulating some of what they were doing, but stopped when I realized that the effort didn't carry over. Most of the design software I've seen looks more like Photoshop than a web browser so the normal canvas/palette work doesn't translate readily into (S)CSS -- I've never (*not since the early 2000s) worked on a project that used a background image so I'm not sure what role the sketch plays in a design/develop life cycle?

I'm not knocking designers or the work you do -- I remember the internet in the late 90s and don't want to go back, I'm just struggling to understand why companies spend so much time/money generating high quality digital art -- that then needs to be translated into images, HTML & CSS?