The SC-MT800 System Information Display shows you what gear you're in, has a battery level indicator, charging port, synchronized shift mode display and also doubles as a Junction A - it has 3 e-Tube ports for you to connect Di2 components to it. The displays lets you charge the system and also connect to it using Bluetooth and ANT+. It is usually installed on mountain bikes.
If you're thinking of using this display on your road bike to replace your Junction A (SM-EW90 / EW-RS910) and Wireless Unit - you can! Note that you can only have one component with wireless/bluetooth functionality plugged in. If you connect both this and a Wireless Unit the Di2 system won't function.
Synchronized shifting on mountain bikes
If you want to use synchronized shifting on your mountain bike the usual requirements apply - you need either a BT-DN110 battery or BM-DN100 battery mount. You will also need a System Information Display. It is not clear why exactly you need one, but we've asked Shimano and they have confirmed that a display is required for synchro shift on mountain bikes.
Shimano also made an XTR version of this component - the XTR SC-M9051. It is identical to the XT version, except for the casing (and price).