Getting Slack messages on my Apple Watch is nice when I am away from my phone but still in BT (wifi?) range. I know this not specific to iPhone, but the integration is nice.
BT if in range, WiFi if not but on a known network. Done that way for power savings.
I love my iPhone, but the ecosystem integration is also a huge point for me. The little ways different Apple products work together adds so much niceness and extra value above the utility of the individual products.
