In my opinion the best thing they offer is little to no wait time even for same day appointments. Beats urgent care experience by a massive amount.
Being able to reliably schedule same/next-day appointments is such a breath of fresh air. You might not see your specific primary doctor if it's short notice, but I've always been able to see someone, and they'll have access to all your records.
Also, having my health record available regardless of which doctor, office, or city (within their network) I'm visiting is nice.