The following is from vim-sensible plugin:
set incsearch
" Use <C-L> to clear the highlighting of :set hlsearch.
if maparg('<C-L>', 'n') ==# ''
nnoremap <silent> <C-L> :nohlsearch<C-R>=has('diff')?'<Bar>diffupdate':''<CR><CR><C-L>
endif
As far as I know, ==# is "always case-sensitive" operator. But what is the purpose to use it when we are compare something to empty string?
And also, what does the entire if maparg('<C-L>', 'n') ==# '' line mean?
:help maparg() explains that maparg('<C-L>', 'n') will return the normal mode mapping assigned to control-L.
When there is no mapping for <C-L>, then it will return an empty string...so it's politely checking for the existence of a mapping before assign the mapping.
Concerning ==# with an empty string, some plugin authors automatically use ==# all the time without considering who it may confuse. :)
empty() in this case which is clearer, though I am indeed among those who use ==#/is# for comparisons almost every tme—the link in the comments is a good one. (Same reason you should use === in javascript unless you really truly know better.)