1. hides any private information anywhere in the request, URL or otherwise, API key or otherwise. Maybe you're fine if someone knows you used Bing (revealed through DNS lookups), but not what query you entered (encrypted to be decryptable only by Bing servers). An API key is obviously secret but something as oft-innocuous as search queries can also be private.

2. disallows someone on the network path from injecting extra content into the page. This can be an ISP inserting ads or tracking (mobile carriers have been playing with extra HTTP headers containing an identifier for you for advertising reasons iirc) or a local Machine-in-the-Middle attack where someone is trying to attack another website you've visited that used https.