That use-case seems not strictly useful, but things become trickier with more elaborate expressions; like when we want to abstract over what f looks like. Eg O

That use-case seems not strictly useful, but things become trickier with more elaborate expressions; like when we want to abstract over what f looks like. Eg One might have f(x,y,z) = (...). If f is in some class of functions with some properties (homogeneous, linear, smooth, etc) we can operate on it abstractly. We could even derive properties of surfaces f(x,y,z) = C.