Engaging advertisements on Facebook benefit from lower acquisition costs because the Facebook algorithm rewards more interesting advertisements with lower CPMs and wider distribution. This is done, as all things surveillance capitalism driven, to keep eyeballs on Facebook.

This isn't too dissimilar to large cable networks that provide free high quality advertising to mass manufacturers in late night slots. The network generally can't sell all of their advertising inventory, particularly in low viewing hours, so they'll offer free or incredibly cheap commercial rates to their bigger buyers (like Coca-Cola or McDonalds, for example) to fill space and have more professional looking advertisements between the low quality advertisements from local mom and pop stores and the "as seen on TV" spots. These higher quality commercials help keep the audience engaged and prevents viewers from changing the channel.