They pass to the task itself, but they show up as the placeholders in the email notificaitons
Just my opinion, but fields are fields, whether built-in or custom, All fields should be available for notifications, filtering, sorting, reporting......it's a database platform it shouldn't make a difference which type of fields are available for which screens or functions.
This is a very logical expectation since this already works effectively for Tasks. That this doesn't extend to Approvals - which are, I think very logically, just a more specific type of "Task" - seems like an oversight from the original enhancement to Task.