Eventually it all becomes about communication between teams of people.

"We just talk to each other" doesn't work well enough when you no longer know who to talk to. Email aliases for teams can help, so that you don't need to know who the right person is for your question, you just need to know the right team.

Things can't live in peoples' heads anymore. Task lists need to live somewhere. Tribal knowledge needs to live somewhere. Priorities need to live somewhere. Some tools may help. You probably eventually will need a bug database. Depending on what world you're in, you may need a requirements database.