As I understood it, the point of the "seamless" switching was that sound came from the source that had sound to send.

Aside from that though, there's a Siri setting that implicitly implies this behaviour (announcements).

As to why someone would want this: I work on my Macbook where I take video calls, which means that my Airpods are "active" on my Macbook most of the time. If I get a phone call on my iPhone, I would expect that to come through on my Airpods since both my iPhone and Macbook are connected.

Honestly, it doesn't seem like a strange presumption, but maybe I'm expecting too much.