An Alternative Host is basically a co-host, but unlike a co-host, they can be designated before the meeting starts in the meeting settings, and they can start a meeting.

The co-host must be designated after a meeting starts and cannot start a meeting.

You should add an Alternative Host if you cannot be there to start a meeting or you want someone to start a meeting before you're available.

Important note: The Alternative Host must have a Zoom account, whereas the co-host does not.

See below for some videos that compare the Host, the Alternative Host and the Co-host.