There are many choices for email client interfaces. HTML for email does not have a good reputation among hackers. After all, email can be considered an ancient technology and is historically based on plain text - HTML breaks not only the philosophy but also many of the tools developed around email.

I have found a sweet spot for an email client between a pure CLI and a full-featured (HTML) GUI client - I use Emacs Gnus, which takes full advantage of Emacs' text-based interface. As always with Emacs, the learning curve is a bit steep at first, but the rewards can be reaped afterwards.

This question shows how far we’ve travelled from the original concepts of sending/receiving/viewing email. I just found it funny that you said “regular HTML client”, as if that was the default interface for email. Originally, it was all text, so this post is in many ways closer to how many thought of a “regular” client. But ever since Hotmail, it’s been a gradual shift away from command line email towards web applications. Desktop GUIs are still (kind of) holding on, but even they are more likely than not to be an Electron app.

To answer your question, these days, I’m not sure. There are so many extra features that email providers (Gmail/Office365) include in their web interfaces, it’s hard to not make the argument that the web interfaces are the better way to use email.

There are times it's really useful to access email from a terminal, and terminals are widely available (shell on your primary system, Termux on Android, SSH to your email host, whatevs).

It's also often convenient to either script interactions, or to have full access to shell tools when interacting with email. I practice this more often with mutt, but I can filter either messages or metadata (headers) and send those to an awk or sed pipeline to extract specific information of interest (this is especially useful with notifications / alert emails). This might be tens, hundreds, thousands, or more messages that are of interest.

Full-blown GUI or Web client email tools are pretty, but lack this flexibility.

I don't have much knowledge regarding mail but I can think of two reasons.

First is the use of mailboxes if your mail provider does not provide you with an IMAP server to connect to in which case you'll use a client like mutt to manage your mails.

Second one is the accesibility through the terminal could be reduced with HTML sites. If I want to access my email through a headless server using lynx or similar having to refresh the website to check new mails, or even composing them might be difficult.