f1 :: [forall a] a -> Int
f1 x = 1
In contrast, existential quantified functions are very useful, because there're always *some* types can do certain things. Use the same example:
f2 :: [exist a] a -> Int
f2 x = length x
I can do nearly anything inside f2, because there's always some x will satify the operations applied on them.
You should notice that only functions or Bottom can have universal quantified type.
Bound helps universal quantification a lot. When you give a bound to universal quantification, you give it extra informations.