为什么服务工作者respondWith()可以返回一个fetch对象而不是一个真正的响应?

我在看MDN文档,弄不明白为什么 event.respondWith 可以有一个 fetch

addEventListener('fetch', event => {
  // Prevent the default, and handle the request ourselves.
  event.respondWith(async function() {
    // Try to get the response from a cache.
    const cachedResponse = await caches.match(event.request);
    // Return it if we found one.
    if (cachedResponse) return cachedResponse;
    // If we didn't find a match in the cache, use the network.
    return fetch(event.request);
  }());
});