The insurance company that provides your ACA plans gets money from the government for doing so. How much money they get is tied to a few things (not an exhaustive list):

1. On average, how healthy is your group of ACA plan holders? If the group has a bunch of chronic conditions, they get more subsidy money to offset the increased care costs. Going to the PCP allows them to have official medical evidence of those conditions.

2. The government gives these plans quality ratings to help people compare them to each other. These ratings are partially based on how often patients get their annual screenings and patient satisfaction. A gift card for a PCP visit accomplishes both aims.

There are also more practical concerns. Preventative care is cheaper than an acute incident for the company. You'd rather catch an arrhythmia at a PCP appointment than pay for the cost of a heart attack.