What it is
Microsoft 365 bundles Exchange Online, Teams, SharePoint and OneDrive, along with device and identity management. The common denominator is Entra ID: nearly every permission in the suite hangs off it.
What it is for
Working across sites without running the servers for it. That has become the default, and the reason is rarely a feature. It is the operations work that disappears.
What to watch
Two things get missed regularly.
Storage moves from a file server with permissions that grew over fifteen years into SharePoint, and the permissions move with it. A migration is the only good moment to clean that up.
And the default retention in the recycle bin is not a backup. If you need to restore from weeks ago, plan that separately.