For small companies and freelance or solo developers, hosting can be an expense which adds up over time to a hefty bill. Whether you have several VPSes or shared-hosting accounts, it makes little sense to have multiple small, experimental, or low-traffic sites on their own servers. You could serve them from multiple subdomains or subdirectories, but yuck! Domains are cheap and I like the polish of having something on it’s own domain once it’s public.