Music, being able to tell what time it is, navigation¹, emergency calls², non-emergency calls/communication. Or, … the use case in TFA.
> If you really need to be reachable or to be able to pay for something in emergencies, that’s what smart watches are for.
… not everyone has the means to drop $400+/y³ on a watch & plan. Smart phone & plan prices are high enough already.
(¹perhaps more useful when I'm biking than running)
(²I've literally witnessed accidents while out on such a run. Having a phone is handy.)
(³we have to restrict ourselves to "the set of smart watches that are capable of making a call independent of a phone, here, too. If the watch requires the phone to make the call … well, that'd defeat the point. This is approximately the per-warranted-year price of a Samsung Galaxy Watch with LTE support.)