Dont ADD up/ Subtraction?

I think you basically think of it as a countdown. So set it to, say 300, and it will go down from 300 miles/km to 0.

Useful if you know you have a certain distance to travel or until a service perhaps. Can't see myself using it really - unless I've got it wrong.

Yeah, seems pretty pointless to me too. About the only use for it I can see is if you know you need to go a certain distance before turning off, but the trip meter will do that too.

Then again, if you're in a region that uses miles rather than km you can set the fuel consumption to litres per mile. Fairly pointless right there too.

The rest of the bike makes sense though!

Perhaps miles/km to any service intervals you may do

That sounds like a cool function (thinking of the TAT) because you can set it to the next major weigh point.
How easy is it to set?