Enterprise features. Want to grant edit access to an online document for specific authenticated viewers using a particular matrix of browser and OS? Click share, edit permissions, send. Want to see if someone is on vacation? Put their name into the "To:" list, it'll show you their current OOO message so you can skip composing the mail. Want to grant read-only access to a particular group of users from a particular part of an AD forest to a specific shared mailbox? Couple clicks and you're done. OIDC auth with MFA so users can use a 3rd party service to index and access mail and files? Click, click, click.
Well it's integration, improved UX, and features that no other product has. Most companies don't have the time and money to implement all the feature requests businesses come up with, but that's literally Microsoft's bread and butter. You can do more with their stack than with anyone else's, guaranteed. (As long as you are using their stack)
