I'm a bit confused about the use of Vim's menus. I have set wildmenu and set wildmode=list:longest,full but I don't understand for the life of me how to invoke and use the completion feature.
Is this feature useful? Why and how? What kind of completion does this do exactly? In other words, what determines the completion list content?
Any tips and example usages would be appreciated.
wildmenu and wildmode are used for command line completion. The simplest way to try it would be with :color <Tab>: the command line is "expanded" vertically with a list of all the colorschemes available on your machine displayed in columns and an horizontal strip that you can navigate with <Tab> (forward) and <S-Tab> (backward).
The behaviour of command line completion and wildmenu are dependant on wildmode.
See :help wildmode and :help wildmenu for more details.