quoted atoms in type specs

[Ryan Zezeski]

Is there a reason to use a quoted atom rather than a bare atom in a type
specification.  E.g., in the reference manual is the following example:

Atom :: atom()
    | Erlang_Atom %% 'foo', 'bar', ...

I also notice this in a the module getopt in the rebar source code.

-type arg_type() :: 'atom' | 'binary' | 'boolean' | 'float' | 'integer' |
'string'.

Why are the atoms being quoted? I was able to compile a module where the
type specification uses bare atoms. Why not declare it that way?

-Ryan