As a freelancer, you might be juggling multiple projects at once.
If you own the project you would create a team, invite team members, assign roles in order to work collaboratively, and be responsible for the billing/costs of all transfers.
If the project is owned by someone else you can join their team, work collaboratively (with access rights according to your role in the team), and all billing/costs of file transfers will be the responsibility of that team's owner.
Please note that billing for each team is kept separate. So, you will have to add your billing information on the new team while creating it.