Okay, since you seem serious with this, I'll explain it to you.
Imaging sitting in front of your PC/TV, maybe playing your favourite game with a controller, watching a movie or just surfing around the web.
Oh look at that, you got a mail!
Push the thumb on the button, open mail app, open mail, read the mail... done.
Wow, that was easy.
Oh, you wanna read a message in your chosen message app?
Push the thumb on the button, open the message app, read the message.... done.
That was darn convenient, you barely need to focus off what you're doing.
Hang on a minute, now I need to login somewhere and use my chosen ID software.
Dammit, do I have to take the phone up and use two hands for typing my password in?
No wait, I don't!
Convenience, that's all it's about.