The app to install wasn't really an issue given any OS with a default desktop came with an email app.

What brought the popularity of gmail was the huge space provided which at the time felt infinite. I still remember the counter that was showing the size increasing seemingly indefinitely.

Yes, that too. I think the initial sell was a 1GB mailbox. Which was an enormous limit at the time. And another thing the ISPs missed. Most had small limits, "mailbox full" was a common thing and you had to download/delete mail all the time which was annoying.