Apple IDs used to be two different kinds of things: one was a purchasing account, originally associated with the iTunes Music Store; the other, a sync/email account associated with MobileMe (me.com), .Mac (mac.com), and iCloud. Apple merged the two purposes, but many people possess legacy accounts they continue to use together in iOS/iPadOS and macOS in those distinct ways.
For families that share a Mac, you can leverage either Family Sharing or a purchase-based Apple ID to allow everyone to access the same apps (or a similar set) across the computer.
macOS is set up to let you create multiple accounts, and each account can be logged into one Apple ID for both iCloud sync and email and for Apple purchases or the account can use two separate accounts for iCloud and purchases.
