If you don't already have COVID-19 just walking around the grocery store w/o a mask without extensive face to face contact is probably fine.Personally, I don't really think the masks do much good unless you are engaging in up close conversation with a group of people indoors. I doubt they do much if you are just going out shopping and maintaining your social distancing. If you're the type who has to stop and chat with everybody you see, you might want to wear one.
If you do already have COVID-19 just walking around the grocery store w/o a mask can infect and kill grocery store workers who do have extensive contact every day.
Masks work if you're sick. They have for years. They work in every other country that is killing the US in their coronavirus responses. If you protect somebody else's mother we can only hope they protect your father.