Male platypuses have venomous ankle spurs. Why are they probably not mainly prey-hunting tools?

Answer: They are a seasonal weapon used mainly in male competition

They are a seasonal weapon used mainly in male competition ✓ — Correct. The venom apparatus is tied to adult males and peaks around the breeding season, which points to competition between males more than everyday prey capture. A shrimp-hunting tool should be useful to females and juveniles too, because they also eat. The odd lesson is that a mammal's venom can be more about mating conflict than about killing dinner.

They are used to stun fish before the platypus eats them — Fish-stunning sounds plausible because venom often means predation, but platypuses mainly eat aquatic invertebrates and locate them with the bill. The spur sits on the hind ankle, not at the mouth or bill where prey is collected. If it were the main fishing weapon, its seasonal male-only pattern would be hard to explain.

They provide year-round defense against predators — A defensive role is possible in a broad sense, but 'year-round shield' misses the strongest pattern. The venom system is functional in adult males and peaks around breeding season, which points to male-male competition as the main story. If defense were the whole explanation, the seasonal male bias would be much less telling.