When we are talking about what we have already arranged to do.
What are you doing tomorrow evening? I’m going to the theatre.
Are you playing football tomorrow?
There is a suggestion that more than one person is aware of the event, and that some preparation has already happened.
I'm meeting Jim at the airport = and both Jim and I have discussed this.
I am leaving tomorrow. = and I've already bought my train ticket.
The time is nearly always given and is usually in the immediate future.
He is playing in the concert tonight.
We are meeting him after the performance.