I have to say I've never used an email client, and never felt the need to really.
Sure, Google changed the game when it introduced Gmail in 2004, but most of the browser-based services like Hotmail, Yahoo, and AOL have barely evolved. In fact, in the 16 years that I've had a Yahoo account, only the interface has occasionally changed.
Change isn't always for the best.
The way they've changed the interface and usability of GMail and Yahoo recently has definitely not been for the better (for me). They've just gone from working smoothly to that modern super duper "clunkiness" which seems to be par for the course these days.
I'm just pleased that in GMail you can still elect to use "Basic HTML", which is basic, but it's super smooth and does exactly what I need it to do.
Yahoo still have the option of using the "old" mail, and I remember a while back reading a discussion about Hotmail having plans to introduce a "Basic HTML" version because of the sheer weight of complaints about their change to ClunkyMail.
I've been using Hotmail since they changed to live.com. I use the desktop client to check my email though.
I hate Gmail, my university changed to Gmail for my .edu account and I hate the interface. I also can't get pop access working. Worked fine before the switch.
I did grab the @outlook accounts I wanted. I like live better though, shorter
My problem is I don't want to use a webbrowser for my eMail, I happen to like my Traditional Outlook
Because Outlook style programs store your email data on your HDD, some people & businesses prefer web based access for the relative privacy from snoopers.