That’s an action you seldom need, but it’s useful for workflows that are supposed to be used with more than one vault. The Get Vault Reference action is there to specify a vault to use towards the top of a workflow (specify it exactly once), and then to use that reference in subsequent AFO actions (instead of hardcoding a vault in those).
For example, I have several vaults, and “Create new quick note” workflow that I want to use with any of them. So I add a vault name list selection on top, then use the Get Vault Reference action to look up the vault by name, and then use the resulting reference as input for any other AFO action that comes below. Something like this:
If the workflow gets more AFO actions, they would all use that same vault reference, and therefore all work with that same vault you selected on top.