personally i just dont see why i should use client still.
Your browser is the most vulnerable piece of software on your computer, and while there are a lot of security measures in place to mitigate this, if a malicious actor manages to pull off a successful mitm (tricking the user into installing their own certificate is one way that this can be done; certain places may even require you to do so in order to access their network), they could inject malicious javascript into your webmail client, among other things, and compromise the whole thing. They could steal your login credentials, and if you were using encrypted email and the decryption was happening client-side in javascript, they can get your private key that way.
It doesn't necessarily have to be a malicious third-party, either. Let's say you're using gmail and a browser add-on for pgp, you have to trust that Google's webmail client isn't just recording your private key so they can continue snooping on your email on their servers.
With a standalone email client, this is not a concern. The client connects directly to the provider' via IMAP/SMTP, and unless your PC is compromised it's much harder for the provider or a malicious third party to steal credentials (if a malicious third-party gets remote access and/or remote code execution on your PC though, it's game over regardless of webmail or desktop client.)
Perhaps more realistically though, there can always be ways in which a malicious web page or browser add-on can snoop on the contents of another page you have open (especially if you are using Firefox with its improper site isolation, and doubly so if you are using Xorg on Linux.) A desktop client adds an extra layer of separation between untrusted websites and your email, though this doesn't matter as much if Xorg on Linux, again.
For the average person this probably doesn't matter I suppose, but it's something to consider.
https://madaidans-insecurities.github.io/security-privacy-advice.html#email