This is an old but interesting topic. For me, and it isn’t really an answer I saw here yet:
It’s because the purpose of my work is to be seen.
Sure validation, portfolio and so is part of it. But my main reason is that I made work for others to be enjoyed.
I see sketchfab as a museum, a stage. Now why do people preform an orchestra for a crowd? Because there are people who want to enjoy listening to it. It’s about the crowd. What do you do in a museum of paintings? You go there to look at stuff, and revel in it’s beauty.
Men enjoy aesthetic things, it gives them a variety of feelings, awe, wonder, longing, love, anger, nostalgia and a whole bunch that don’t even have names. As an artist I see it as my purpose to be a supplier of these artwork. To create a vessel, a template for people to put themselves into it. And for that I need to put it in a place where people can find it.
For me sketchfab is perfect for this. It’s takes advantage of an actual 3D object unlike a render does.
Sure not everyone has this as their main drive. Some do it mainly for validation, to test their works again the success of others. Some do it for fun because they like to play around with the tool. And many more reasons. But for me, and I’m sure others, it’s because it’s made to be seen.