When you start a new account, you're automatically the one and only owner of that account.
Imagine for example you created this account for your client, or you want your colleague to be the owner (the name of the owner will also appear in the created invoices if you go for a paid plan), you can transfer ownership.
Go to 'users' in the menu:
Choose (or add) the user you want to transfer ownership to, click on the arrow, and click on 'transfer ownership'.
Type TRANSFER and click on 'Confirm'.