Because many users often enjoy using a dedicated application than a website. Plus it gives developers access to even more customization than browsers normally provide.

If they customers didn't like using it, companies wouldn't keep making these apps.

Personally, I'm a techie guy but I get exhausted with the number of tabs i have open at any time. I don't need to have more dedicated to just slack, Spotify, discord etc

Doesn't work, there are significant functional differences between the apps and the sites

I mean, you could be using a regular email client and a regular webmail app on the browser... the fact they insist on a custom desktop app suggests the main reason is that they really want to cut those out.