Initially, those fields contain "Search" and "Message ID" so we know what they're for.

But, if you actually use them, one or the other may be left blank so there's no idea of their purpose (see attached image).

FWIW the blank Search field was the result of a search that didn't return any results. I wanted to go to the specific message 451431 and entered in the wrong field. After that I put it in the Message ID field to get where I wanted to go.

UPDATE: I see there are explanatory mouseovers which are better than nothing, but not ideal. Maybe some logic that if the field is blank, put in the initial values (?)