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The first answer is that its purpose is to act as a prefix key to the keys that follow it; that is, to serve as a keymap for a certain number of key bindings. Those bindings are also shown in describe-key. Seriously; this is the purpose: to group those keys and their commands. Now what that grouping might be about is another question (see next).

Beyond #1, yes. A prefix key is bound to a keymap, and that can have its own doc string. It is up to the creator of the keymap to provide a reasonable and useful doc string.

To see the doc for a keymap, use C-h M-k (describe-keymap), from help-fns+.el.

Here is what you see for bookmark-map, for example (manuals here is a link to the doc in the Emacs manuals):

 bookmark-map
 ------------

 For more information check the manuals.

 Keymap containing bindings to bookmark functions.
 It is not bound to any key by default: to bind it
 so that you have a bookmark prefix, just use `global-set-key' and bind a
 key of your choice to `bookmark-map'.  All interactive bookmark
 functions have a binding in this keymap.

 key             binding
 ---             -------

 d               bookmark-delete
 e               edit-bookmarks
 f               bookmark-insert-location
 g               bookmark-jump
 i               bookmark-insert
 j               bookmark-jump
 l               bookmark-load
 m               bookmark-set
 o               bookmark-jump-other-window
 r               bookmark-rename
 s               bookmark-save
 w               bookmark-write
 x               bookmark-set

But some keymaps have a more rudimentary doc string. The keymap ctl-x-map, for instance, is general-purpose, so there is not much that can be said (beyond #1 above). This is what you get with C-h M-k ctl-x-map:

 ctl-x-map
 ---------

 For more information check the manuals.

 Default keymap for C-x commands.
 The normal global definition of the character C-x indirects to this keymap.

 key             binding
 ---             -------

 C-@             pop-global-mark
 C-b             list-buffers
 ...