You do not need a Bungie.net account but I recomend that you do create one. All dlc can be bought on the Steam or console store and any future seasons can be bought in game. However a Bungie.net account allows... You to change kit and manage characters outside of the game. You can earn/obtain codes that to be used need to be on Bungie.net. Activities in game earn you discount codes for select items on the Bungie store and you need Bungie.net to obtain them. Most of all you need a Bungie.net account to get full access to the forums, be it for help and advise for issues you have, finding others to play with, clans or general game gossip.