haskell - Why do Maybe/Optional types use a Just/Some type instead of the actual type?

Question

In Idris, the Maybe type is defined as followed:

data Maybe a = Just a | Nothing  

It's defined similarly in Haskell:

 data Maybe a = Just a | Nothing
     deriving (Eq, Ord)

Here's the ML version:

datatype 'a option = NONE | SOME of 'a

What are the benefits of using Just and Some?
Why not define the type without them?

example:

data Maybe a = a | Nothing

Answer 1

What would then be the difference between

Maybe a

and

Maybe (Maybe a)

?

There's supposed to be a difference between Nothing and Just Nothing.