If you drive in Cab mode with this enabled, it will manage the injectors and stoker for you, keeping the boiler from running dry or the firebox from burning through. This will allow you to concentrate on driving, without also having to manage the fire and boiler.
It isn't the most intelligent of creatures though, so if you are someone who likes getting the absolute max out of the loco by manipulating water and coal levels throughout the run, be aware that you can't really do that while it is enabled.
It's not specific to the T1 - any steam loco should have it. It's also been around for a while