Use case: I am doing my run and I see a friend running in the opposite direction. After uploading my run, I can virtually say “hi” to him in the Strava app.
In the app you see who ran in your area at the same moment. They are listed in the app and you can pick who you actually saw.
It is similar to tagging people with whom you ran, but this is more for people who you saw during your run but did not join you. Obviously, in your privacy settings you can configure if you strangers can say “hi” to you, only friends or no one at all.