Each action should have description associated with it, telling what the action is for.

Why? In the big IDE, there are several hundreds of actions many of which only make sense in the context of given module.
In menus, keymap panel and quicksearch, users come across many different actions purpose of which is often very unclear.
They are often disabled, but it is not obvious why.

These descriptions could be used: 
   1) In the new keymap panel (see #123467)
   2) To be displayed in the status line whenever the action is rolled-over (see #140540)