It's a way of namespacing commands—if you install NERDCommenter, for example, <leader>cc comments out the current line (whereas cc by itself changes the current

It's a way of namespacing commands—if you install NERDCommenter, for example, <leader>cc comments out the current line (whereas cc by itself changes the current line). It's traditionally bound to backspace, which is actually-free, but many people bind it to keys that are free but never used personally.