Is a lockdown, or partial lockdown, legitimate?

Does it really help? Or it helps only during the lockdown and then the virus comes back the moment ot ends?

Are mask useful? Should people be mandated to wear them? Are you a danger to others if you go around in restaurants or hang out with people because even if you say "ok it's my responsibility, I take the risk and if i get the virus tough shit" you can still infect other people regardless of if you want or not, so there are a lot externalities.