Control and branding, mainly. In my own business I frequently need to stage sites for customers who either don’t control their DNS or have to go through corporate IT or some other process to set up a subdomain. I can add a subdomain to one of the domains I own in a few minutes because I own and control it. That keeps the project moving and shows the customer I can remove obstacles. Subdomains and DNS may not seem like major obstacles but with many companies just asking for a DNS change is a days or weeks long hassle.