two main reasons:

1. its a replacement controlboard for when you need a way to interact the miner with additional sensors or actors, like a temperature sensor, or controlling an additional fan, pump etc. In the past the real only way to do this was to have a RaspiPi that is connected to the sensors and actors and then talk to the miner via the API. this is unreliable though
2. You want to run on your miner more than just the firmware, libreboard will allow you to run homeassistant, a full bitcoin node or anything else right on the miner.