Some keywords cones to mind based on my own feelings;
- software tribalism and identity signaling to peers. like a mini resumé.
- idealism and dreaming, the cool stuff I wish I was using (Who sticks a .NET logo there, right?)
- added uniqueness, my laptop is mine, my special tool, it is set up as an extention of my unique set of skills
From my impression and experience, it‘s like saying „I am root. This is MY device“ more than anything else. So I‘d say your last point applies in most cases. Although Salesforce-people seem to have a special relationship with their stickers [2]. They have tons of them, but usually none from other companies. They seem to be achievment-badges.
I can spot mine easily at a distance, and it's very unlikely that someone else will have theirs marked similarly. Useful in situations like security lines at airports (Please step to the side sir, while we pat you down and someone nicks your laptop from the baggage scanner), or even in an office where everyone has the same laptop.
But yeah, mostly what you mention.