Pre-existing models/art is something that is a huge work effort. Not to be undervalued. If one can get those for free, it can be the reason some game exists.

Take Auto Chess for example. I can imagine programming that DOTA 2 mod was an effort one or few programmers did as a hobby at first. If they would have had to either pay or network with artists to create the art and other people to do marketing, it would have been a lot more than a hobby.

One of the best games there is, but without any keyboard shortcuts, that will be annoying really soon.