You are right, I was thinking about the fact that have been years that I am using Gmail via web, and found no reason to go back to use a desktop client (apart from gpg support).
Updated question, thanks.
A few reasons for me:
1. Google keep messing around with button placement in their UI and it gets irritating after a while.
2. I would never compose a long (important) email in a web browser in case I lost the page.
3. Gmail is one of many accounts that I have and I like a unified inbox.