Comment by sofixa
Having a bunch of different domains can serve multiple purposes.
In GitHub's case, they already have githubusercontent.com to avoid serving untrusted stuff from their own github.com domain.
Sending marketing or security scanner (potentially very spammy) notification emails from separate domains can help with reputation too, to avoid your main domain getting marked as spam.
These are all legit; Amex having 20 different of domains, half of which smell like phishing, and still sending emails from other domains is just incompetence. Something like marketing people or someone dealing with strategy deciding to do stuff in a certain way, with nobody technical in the room to tell them why that would be a problem. As an example, a friend of mine's organisation wanted to do a SaaS website for their niche, and a separate website to advertise the SaaS (separate domain, visual identity, everything).
My theory for most of these cases: they would need permission from who knows what department(s) to set up a subdomain of the main domain for their project, and it's easier to just purchase a new domain for the team/project.