Why is an identity table required, but property and event tables are optional?

The identity table is required because it contains the native and channel identifiers required to send messages to customers. The property & event tables are "nice to haves" if you want to add personalization, but you can't send anything or sync a list of customers without the identifiers required by the channel action.

If all you need to do is send data to downstream channels, you don't need to include any property or event tables, however if you want to build segments (and not just for personalization), you need to add some property or event tables otherwise all your segments could be based only on identifiers.