Facebook Business Manager is a platform that helps you manage all of your ad accounts and Facebook pages in one place.
If you have several ad accounts or if you have other team members or app partners that need access to your ad accounts, you will have to create a Business Manager account and then add your ad accounts into your Business Manager.
We'd suggest you create a Business Manager account before running Facebook advertising. Once you switch to Facebook Business Manager, you will not be able to access your ad account from your personal Facebook page anymore; however, you can manage all of your ad accounts and pages with related access permissions easier here.