Health insurers are limited by law as to profit margins. So how to make more money? Raise prices, or signal to providers that you'll pay higher. Because if your incoming premiums have to rise, then that percentage that can be captured as profit rises.
But wait... what if you (an insurer) build/buy a middle-man to route prescription money through? That isn't covered by those profit margin constraints. So you can just up the prices of prescriptions and siphon profits that way.
Even better, you can do entirely sketchy BS (looking at you, Aetna, but also others): "Sure, you can get your scripts filled at your local pharmacy... but only for <=30 day supplies. We'll reject any script authorization for a supply of 31+ days, like those extremely convenient 90 day refills... unless you use the mail-order pharmacy that is wholly owned by us", thus making people choose between convenience and pricing.