Your "why" is exactly what selykg answered. To paraphrase: there are some people that would like the convenience to have calendar and e-mail client in their browser, just because it's already there and maybe intersects with the workflow. A niche? Maybe, but there's always someone.
In the 90s, the Internet was seen differently. The core functions - email! Shopping! Message boards! - were thought of as the pillars of what made up the World Wide Web. And so AOL and Netscape and others bundled the software into suites, where you got all your Internet stuff done. Mozilla used to be this way in its early days, before the web browser branched to Firefox.
Opera did it, too, and Vivaldi is a spiritual successor to Opera.
I switched to Vivaldi on Android because I didn't want to use Chrome, but Firefox on Android was doing two things that annoyed me: Reloading tabs when I navigate back to them after a long time ( which is a delay at best or loses the page state at worst) and as of six months ago, the screen would stutter when scrolling through pages like the New York Times.
Anyway, I tried Vivaldi mobile and liked it, then decided I might like the sync features between desktop and mobile, so I now use it on desktop.
And then I decided to try their mail client, which "just works" for me and my light usage. I already have the browser up all the time, so I hit F4 and check my mail.
Mozilla was like this too because it was (initially) the open sourced Netscape code, at least until they decided to throw most of it away and start over.
