why feature collect needs defer work function

[chst1]

Hi！There are following codes in collect function:

if (!executors.empty()) {
    auto work = [](Try<typename decltype(future)::value_type>&& t) {
      return std::move(t).value();
    };
    future = std::move(future).defer(work);
    const auto& deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }

https://github.com/facebook/folly/blob/main/folly/futures/Future-inl.h#L1476 I have a question why add defer work function. What I think is that is same to add a then callback while defer work function. Normally, when Context is destructed, feature will start process, adding work function process will become ~Context -> precoess work -> feature's callback. why do we need add work func rather than processing ~Context -> feature's callback directly.