May I ask the opposite? Why use JavaScript client from the web instead of desktop ones?
Most operating systems, excluding Windows, are shipping with decent native and fast email client. They are automatically updated with the system, again excluding Windows, integrate with other apps (for ex. right-click and share with mail), can store messages offline just in case and are overall nicer to use.
The only use case I think of is when using someone’s else computer and you don’t want to remember to log out, because browsers have “incognito” mode.