It does exactly what I think it does.

It makes sure that there is no way to get an error "subquery returned more than 1 value".

It should not be happening anyway, because the record is filtered out by an exact match to a PK value.

Only 1 record should be returned.

Therefore ORDER BY would be only an unnecessary overhead.

Unless the table definition is changed.

Then the logic of the query (probably not only this one) would require revisiting.

But while developers working on it the TOP 1 would prevent run-time errors in Production.