What is the incentive for the bot owners? Why do the bots download and install the apps? It has some (very small) cost to them, and I don't understand the value for them.
One reason: It helps makes the bot accounts look more legit. It's why Facebook bot accounts like random legit pages.
upping the clickthrough rate on a low quality property to get paid for fake ad impressions?
The bot owner is also providing "ad space" to Google. Author pays Google to show the ad. Bot owner gets paid by Google to "display" that ad and for conversion. The bot themselves aren't getting paid for installing the application.
In a normal scenario, the provider of ad space would be a website or an application. Google would pay the owner of that space. On Youtube, that would be an ad served by Google paid to the channel owner.