1. Extra revenue stream
2. To be able to sell their devices in countries not supported by Google Play (China).
3. As a leverage to get better licensing deal from google. Manufacturers must pay Google to include Play Store in their device, and it can cost up to $40 per device. By having their own app store, phone manufacturers might have some leverage in licensing cost negotiation (e.g. Google offering them money to not include other app store).

Samsung has one and they aren’t allowed to have it as the default.