Why have both variable and function concepts in C++ Concepts TS?

I've been looking at the C++1z N4377 Concepts TS draft that is being implemented in GCC 6, and I don't understand the purpose of having two different kinds of concepts: variable concepts and function concepts.

The relevant part of the draft for function concepts is [dcl.spec.concept (5.4)]

• The declaration shall have a function-body equivalent to { return E; } where E is a constraint-expression (14.10.1.3).

and for variable concepts, in the next paragraph [(6.3)] :

• The initializer shall be a constraint-expression.

Is there anything one of them can do, that the other cannot? If not, is there a rationale for including both?

_{Note: the latest draft, P0121R0 doesn't change anything in this regard}