Who else but Mother Nature herself? That is to say: nobody. Evolution by natural selection "chose" this design for this "reason."
Is it unwise to speak this way? I call this the problem of free-floating rationales. We start, sometimes, with the hypothesis that we can assign a certain rationale to (the "mind" of) some individual creature, and then we learn better; the creature is too stupid to harbor it. We do not necessarily discard the rationale; if it is no coincidence that the "smart" behaviour occurred, we pass the rationale from the individual to the evolving genotype. [The Intentional Stance, p.259.]
The benefit of doing so is that adopting the intentional stance towards "Mother Nature" offers great predictive power, and a robust-process explanation of why animal genotypes contain the innate information that they do. It's no coincidence that ants remove corpses from their nests. Indeed, if the decay process had released some other chemical than oleic acid, we may be fairly confident that the ant "funeral march" behaviour would have evolved to occur in response to that other chemical instead. This then means we can attribute a sort of intentionality to Nature. The ant behaviour has a function, to fulfill a particular goal (i.e. removing dead nestmates) -- but it is a goal of Nature, rather than of the creature itself. If practical intelligence is the ability to produce goal-achieving behaviour, then the intelligence behind the ant's behaviour is not the ant, but evolution.
Now, we can ascribe a weak sort of intentionality on the basis of biological functions. There is a genuine sense in which the ant behaviour is about removing dead nestmates. Evolution selected for a behaviour to fill this functional role. But we are also interested in a stronger sense of intentionality. Sometimes we want to attribute intentionality to the animal's mind, rather than Nature's. When are such attributions warranted? When it is the animal that recognizes the link between the behaviour B and the goal G; when it is the animal that engages in practical reasoning.
This is where decoupled representation becomes vital. Without a separation between indicative and imperative representations, between stimulus and response, there is no room left for practical reasoning to occur. Thus decoupled representation is a necessary precondition for fully-fledged intentionality in animal minds and behaviour.
It's worth noting that intentionality can survive a separation between nature's goals and the animal's own. For consider sex. The biological function is obviously reproduction -- having sex just happens to be a good way to make babies. But human sexual behaviour can be intentional even if the person doesn't recognize the biological goal. Rather, they might take sex itself as their goal, and engage in practical reasoning about how best to achieve it. This will still require decoupled representations, of course. My point is merely to highlight that one cannot infer non-intentionality merely from ignorance of biological function. The behaviour might be intentional under some other goal description.