Comparing the lists with == requires elements to be comparable (denoted as Eq a).
Prelude> :t (==[])
(==[]) :: (Eq a) => [a] -> Bool
For example, [sin] == [] won’t work, since you can’t compare functions. It might seem stupid, but the type system must judge type of an expression without looking at its value.
An alternate check would be length xs == 0, this doesn’t require equality but won’t stop if your list is infinite (try length [1..] == 0). That’s why there’s a dedicated function.
null [] = True
null _ = False
Prelude> :t null
null :: [a] -> Bool -- Notice lack of (Eq a).