Microsoft is looking to unify all its services into a single app and eliminate the old desktop client. This will allow Microsoft to have a shared experience across platforms without having to deal with differing code bases. This will also help Microsoft to make the apps available to all the platforms easily.
As per the plans, it has been named Project Monarch and will replace all Microsoft email clients, including the default Windows Mail app. Project Monarch will be built on a single codebase and will be accessible to all customers, businesses, and consumers. Currently, Microsoft has a number of different clients for desktop, including the Outlook app for Windows (Win32), Outlook Web, Outlook for Mac, and Mail and Calendar for Windows 10.
While you can technically install Outlook as of now, it doesn't have all of the integrations that Monarch will have. For example, Monarch will include support for offline storage, notifications, and share targets. Microsoft wants to ensure that it feels "native" to each platform while still adhering to the same look and feel as the Outlook website.
However, it will be slightly different from the Outlook app already available as Progressive Web App (PWA). Microsoft wants to make the app as native as possible, yet keeping the app's look and feel similar across all the platforms. The leaked app is in an early stage, and Microsoft has said that it doesn't include an offline mode for now. The upcoming Outlook client will be getting into the preview program by the end of 2021 and will replace the default Mail and Calendar apps on Windows 10 in 2022.