The customers of hotels and airlines are, by definition, travellers. That means that their "computer" is very often at home, on their desk, switched off and inaccessible, while they are travelling. Many of the functions that are of use to a traveller - the ability to check in, download a boarding pass, change flights, etc etc, are of most use while they are on the move and away from their "computer". But when that "computer" is one that most people carry around at all times - their smartphone - then things change. There is more chance that a passenger will be bringing their smartphone on board their next flght/on their next hotel stay, rather than lugging their desktop and monitor onboard/into the hotel lobby!

If you're an airline or hotel company, you want to be "accessible" to your customers at all times. That means focusing on the "computer" that they actually carry with them at all times - i.e. their smartphone.

(Or imagine it this way. It's the equivalent of having a landline and a cellphone. Why would that same airline or hotel company, knowing that you are currently on the move, contact you only on your landline number, when they instead have a cellphone number for you? Your landline is not much use for contacting you when you are out of your house, just like your "computer" is not much use to you when you are away from your desk).