The problem here is that the quality of the apps not always high enough and we can get some troubles due to the app bugs.
What if the person is not having a computer but rely on phone for gambling? Did you know that most people that are accessing gambling sites these days are using mobile phones far more than computers? Some people even do not have computer at all but are able to buy high end phone and use it for browsing and including using it for accessing gambling sites? What if the gambling site can develop a desktop app too?
The same time, using the phone is much easier today than using PC. For gambler it can be the most important thing - possibility to bet any time he want, not waiting while he can reach PC.
So, if the casino can develop a high quality app - it is what they have to do, but it must be really the good app. In any other way - better even don`t try to do it.