When you buy software off the shelf, you’ll use it for a number of years, and then decide that it’s too old. It’ll get creaky and slow, and the formats it uses will become obselete. It won’t be supported on newer computers, and newer versions will become available.
Eventually you’ll upgrade.
The great thing is that you do it all at once, at your convenience. You get confused for a week as your favourite features move somewhere else, but you get used to it.
Web applications are updated when the engineers decide it’s appropriate. OK, you don’t have to pay for the latest version, but what happens when a critical feature gets moved when you are racing towards a deadline. You’ve got 3 hours to finish a large project, and suddenly you can’t find the “Bold” button. (I’m exaggerating slightly). I don’t think this is a good thing.