There’s very often a comparison to the somatic (i.e. non-cancer) genome of the same patient. It’s a great way to quality control that there wasn’t some sample mixup in the lab.

Transmission of cancer is rare in humans—if it were not, it would make someone’s career to find many cases of it. While we can’t say that all sheep are white, we’ve looked at enough of them to say that black sheep are not common. Furthermore, it’s very clear how the Tasmanian devil cancer is spread—it’s around the mouth while they are biting each others faces; it’s not as obvious how one would spread most human cancers.