The point is that it's a more efficient way to travel. Both in time (you travel in your sleep), and energy/pollution. And it's more comfortable (or in this case luxurious) than the cattle-drive experience of air travel.
It is more expensive, though. But that's also because air travel gets lots of subsidies and doesn't pay for externalities. That should change.
It's not necessarily more expensive either. Instead of arriving in the afternoon, you have the whole day ahead of you, and you mostly spent down time for travel. To get the same amount of waking hours in a trip where you fly you would need an extra night in the accommodation of choice at the travel destination. That's €50 – €100 extra. And of course the costs of getting from the airport to a place you actually want to spend time, and the costs of your time in terms of leave taken from work!
For many sleeper trains already are cost effective. And as soon as governments finally give in and stop subsidising air travel it will only become more attractive.
And every train fan knows that there is one thing that can't be beat: get off the train in the early morning, take a few steps, and you're standing in front of the station and you're done. You can go visit a museum or do some other leisurely thing. No security, no delays in getting your luggage, not being stuck in airport getting ready for the train/taxi/bus to some place humans actually want to be.
Indeed, when comparing trains to airplanes, the time necessary to get from the city to the airport and the security checks & boarding time are generally forgotten (repeat at arrival). It's generally around 1 to 3 hours for a single flight.
So, for a short flight, using the train may take the same time than using the plane (particularly true in Europe with the speed trains).
For example, Paris-Frankfurt, next wednesday with Deutsche Bahn ICE 9553 it's 4 hours for 56€ from city center to city center (cheapest price for the week, it can go up to 350€). By plane, with Air-France it's 1h20 from airport (CDG) to airport (FRA) for 127€ the same day. From Paris center to CDG airport, it's around 30 mins, and for Frankfurt airport to Frankfurt center it's 40 minutes. Both ride by local trains (for 5~10€ each). All in all around 2h30 from city-center to city-center, but then you need to not forget the necessary time for security check & boarding. Let's say 1h30 only because you have no luggage... If you have a luggage add 40 minutes more.
All in all, for this ride, the time is around the same, but the stress is not...
Unless you actually want make sure you're on time for the meeting. Unless you are in Switzerland or Japan, if the train is supposed to arrive at 8am, the chance you'll make your 9am meeting on time is a coin toss at best, even if it's held right next to the train station.
Also the hotel is guaranteed to have a much more comfortable bed, a shower and a better breakfast.
