Just a resident, but for somebody going into employed practice I think it's not so much business knowledge as knowledge about how reimbursement works and how to maximize that. Knowing how to document in a way that gets you a level 4 visit when it's deserved instead of a level 3, which these days really doesn't take more than an extra sentence or two in your note in most cases. Double billing when you manage an acute concern or chronic illness at a wellness visit. Knowing you can bill for things like a PHQ9 or an MCHAT or vision/hearing screenings at certain visits. Properly billing for procedures (and maximizing your office-based procedure knowledge during residency so you have an opportunity to bill for these). A good FM residency should be teaching you those things. Having a good relationship with an office coder/biller who knows what they're doing can help you with those things as well - they should be sending you emails to say hey, if you document X then we can make this a higher level visit, for instance.
You really only need the business knowledge like dealing with payroll, HR, hiring/firing, purchasing supplies/equipment, etc. if you want to own a practice, otherwise the health system will take care of most of that stuff for you.