Once you have linked Online to your Nintendo account, you just turn on a previously grayed-out config option and from then on it works automatically for all compatible games. You can go and change the settings for individual games.
Aside from the basic assurance that you don't lose your progress if your Switch is broken or stolen, and it being handy for the bigamist who keeps a console in each household, I've found practical use for it on a couple of occasions. If you know you're going to come back to a game eventually but you need to free up the space and you don't want it clogging up the home screen, it gives you that.
Once you have linked Online to your Nintendo account, you just turn on a previously grayed-out config option and from then on it works automatically for all compatible games. You can go and change the settings for individual games.
Aside from the basic assurance that you don't lose your progress if your Switch is broken or stolen, and it being handy for the bigamist who keeps a console in each household, I've found practical use for it on a couple of occasions. If you know you're going to come back to a game eventually but you need to free up the space and you don't want it clogging up the home screen, it gives you that.