If you own a mobile device you can download Microsoft Outlook for Android and iOS and you wil have an app capable of sending and receiving mail for multiple accounts and connecting to your contacts and calendars whether you choose to use live.com, icloud.com, google.com or some other cloud system.
If you own a desktop, that version of Outlook is far less capable. Essentially its a mail app because the calendar & contact functions won’t connect to the cloud meaning you can’t see them on your other devices.
These two apps seem to have been developed from different origins. The mobile app was developed as Acompli and simply bought and rebadged. The desktop app has a much longer Microsoft lineage. The difference shows — mobile grew in a cloud based world where everything connects and syncs with everything; desktop doesn’t seem to have expanded beyond the “personal” in PC.
This basic difference means the desktop app, particularly on a Mac, is completely unsuited to a multi device world. Until Microsoft gives it solid cloud capbability, don’t waste you money.