I am certainly going out less than I would normally, but with the mask do I go out more than I would have otherwise? Hard to tell. I will say, when I do go to a store, it makes me more comfortable. Apart from giving others a bit of assurance that I am not spreading it to them, I myself feel better about my own health because ...
I am much less likely to touch my face. I am sure that I would do it numerous times out of reflex without the mask. As soon as I get in the car and mask goes off, the Purell goes on. Not perfect, but better than nothing.
Someone posted here on TUG a link to an article that suggests that the amount that you are exposed can affect how badly you contract the disease. (With a lower level of exposure, your body has a better chance of getting a jump on the disease.) If so, while the mask does not give you complete protection, it does reduce the degree of your exposure.
Finally, an amusing story. A friend was walking around in a residential area of the local town. Someone sitting on a porch coughed. They looked up and said "Don't worry, cannabis, not coronovirus."