You discovered something that isn't documented so it opens up the opportunity to think about use cases.
I typically work in the supply chain:
Here's how I might use it.
Set a Boolean to check if an item was ever out of stock during the year.
As we know, Booleans are fastest and most efficient data types to work with.
Use the Yearvalue to filter on the items that had at least one stock out during the year.
Do missed sales analysis on those items.
To be honest, I can think of a lot of use cases. Like counting how many items had stock outs during the year, number of vendors that had late shipments, receipts that took longer than a normal leadtime, forecasts that are below the lower control limit, above the higher control limit....
A often used tool in my models is the ability to select a year via a boolean check on a dashboard. This line item is formatted as a year time frame which feeds a time property model to allow the user to filter for that particular year. Without YEARVALUE() the time property module, dimensioned by date or month time period will not know where to go to find the condition.