Why does "->" exist?

[not.here.now]

The C compiler knows when something is a structure and when it's a
pointer to a structure: Why can't I write "foo.bar" in either case and
have it dereference "foo" as many times as necessary? I often change
my mind about whether e.g. the argument to a function should be the
structure itself or a pointer to it, and it would be nice if I didn't
have to change all the code that accessed the members.

I read that article by dmr on the history and development of C, and he
talks about a time when "foo->bar" would work "almost" regardless of
the type of "foo", with "bar" just describing an offset and the type
of the member, without any particular tie to the structure type -- but
this confuses me; was it not possible to use the same member name in
two different structure types, at least if they were going to be at
different offsets? Anyway, even if there's a historical reason related
to this, why was the division kept after the type system got stronger?
Backwards compatibility would have been preserved by generalizing "."
and deprecating "->".

I presume there's a good reason that I'm just missing; can someone
tell me what it is?