I would imagine that it's free as MS is likely to launch lots of Microsoft 365 AI/ML functionality for email soon and they'd like you to use Outlook app on iOS and the Mac to benefit from this (subscription $).
(I imagine that they'll launch the new Outlook PWA app to all users on Windows 10/11 at the same time).
It's not a bad application, though as a consumer, I think it only makes sense if you're using hotmail.com or outlook.com, as the spam, phishing and scheduled delivery functionality hooks directly into those cloud mail services.
As to why you'd use this with iCloud mail or Gmail (or quite frankly, any other email service) would be a mystery.
The custom UI elements that they've done makes it seem slightly slower compared to Mail.app on my machine - as well as making Outlook absolutely feel like a port from another OS (arguably it could've been made more Mac like and still looked like part of Microsoft 365).
It also does the dubious job of making Mail seem like a great native and speedy Mac app (when arguably, it isn't).
All in all, for something so ubiquitous it makes me think that native macOS email clients are not exactly in a great state (and no, I don't want to try one of the smaller 3rd party apps that download your email to their servers to work their magic).