First of all there is the professional aspect of it. People just have to market themselves to get jobs. ArtStation (its predecessors actually) eradicated the new to have a portfolio website. It's THE place to host your work.
Then there is the need for validation. There is nothing wrong with wanting to be appreciated for your accomplishments. That's just one of our needs.
Community aspect is also important. People want to get better and they want to learn from others. Showing your work and exposing it to scrutiny or inspiring others is perfectly fine.
Lastly, I've never said that we derive EVERYTHING from the process. Final result is also important. It's just meaningless without the process, imo.