why "." vs "->"

Im an old, emphasis on old, K&R guy, so I am very well versed in accessing members of structures, unions, and now classes with "." notation vs "->" notation.  I was asked this morning "why the difference?"  My answer started with "That's easy," and then every instance that I could come up with I ended up deciding that a modern compiler could easily overcome the issues.  

So, given a structure or class foo with a single member "a"

I understand that
       struct foo *pfoo, afoo;
       pfoo=&afoo;
       pfoo->a would be the appropriate call

OR
      struct foo afoo;
      afoo.a would be the appropriate call

but why are they separate.  In case 1, why can't the compiler sort out pfoo.a or in the second afoo->a.  There must be a case where this behavior would be unacceptable, but I am trying to fathom what it is.