It did used to be that with a new business, especially one where you have competitors that might try to buy the adjacent extensions to redirect to their own sites, that you locked up at least the .com .net .org , but this isn't standard practice these days, probably because people are more internet savvy and don't put in the wrong extension too often.
For example yahoo.net and yahoo.org redirect to yahoo.com but uber.net and uber.org go nowhere.