Either only to this mail or to both this and the main email on the account. It doesn't matter.
It's for us working in bigger corporations where we need to manually login to DigitalOcean and send invoices to the financial department. This is a pain in the ass, and I don't want to give financial access to our servers either (you cant allow them only into the billing).
On other services you can create seperate accounts that can only access billing information, or only servers or both. And you can also specify alternative mails to send newsletters, security netters, general information and invoices to, so they can go to the relevant people in different departments.
I'm the CTO of a subcontractor for a large company, and I've had to make an appointment in my calendar the 1st of every month to manually log in, collect the relevant data, and send it to accounting.