a) It's about letting people resume their activities and keep a low R0.

b) It's not 0, so there's still room to reduce the number, even now.

It's about maintaining an R0 < 1 after social distancing is relaxed. Without contact tracing we're destined for a cycling of lockdowns until we have a vaccine.

if you re not social distancing there s no way you can stop this virus with an app. it's a highly highly infectious airborne virus. you can ease the lockdown and keep distancing rules. not sure if the app will help in that case