Yet another thing that archivists suspect is under-appreciated by, if not completely unknown to, the general public, is the central importance of cataloguing to what they do. (Even if is also the activity that tends, frustratingly, to be the one that gets pushed down the priority list by more immediate demands.)
One still finds people who make an initial contact, or just come in, and think that the way to engage with an archive is to start with the first box and go until they reach the last file in the last box.
A good catalogue performs the function of the London Tube map in providing an overview and showing how the items within the collection are related to one another. It will probably also include helpful background information on the person or institution and a general account of the collection, what it contains, notable gaps, state of order when found, etc.
A recent post by a colleague on the Wellcome Library blog draws attention to the importance of the contextual information about the individual item that the classic archive catalogue provides, and the impact of new online approaches, their advantages and disadvantages.