Most people (mainly big businesses) will register as many different extensions as popular to protect their brand. Think about YouTube for example.. YouTube doesn't want someone typing in YouTube.net only to hit a parked page with ads on it..Yet, hundreds or thousands of people probably do it everyday. By registering every extension it gives them the ability to forward all that traffic to their real website.
Most new businesses don't really need to register all extensions unless it's some kind of short or brandable name (in my opinion)