Apparently the CDC recommendation is not a CDC recommendation but a recommendation forced on it by the White House, the basic worry is that people will reduce or eliminate their social distancing due to a false sense of security and thus risks increasing infection.
Some public health experts at the US Centers for Disease Control felt "pressured" by the White House to draft recommendations that all Americans wear masks or facial coverings while in public, according to a senior federal health official involved in discussions.
A source told CNN the "CDC was under intense pressure to do this quickly."
""The CDC would not have gone this direction if not for the White House," the senior official told CNN. "We would have tried more to understand about asymptomatic transmission. We would have done more studies if we had more time." "
"It was more than we were comfortable with, the source added. "Things were done very fast. It's a credibility risk to the agency. It's not fabulous for credibility. There is a real worry that this will provide a false sense of security to the public. Also, up until now the CDC has been telling people it's okay not to wear a mask, and it's going to be recommended that we do. It runs the risk of confusing the public."
https://www.cnn.com/world/live-news/coronavirus-pandemic-04-03-20-intl/h_8f7ec40bf3b90b0099bf36fa5b1cc68bAs to the science behind whether wearing a mask helps, arstechnica did a brief literature review. N95 when used properly helps workers prevent getting infected. Surgical masks seem sufficient to help prevent spreading infection to others, but not from getting infection. Cloth masks likely don't prevent (at least hospital workers) from getting infections and actually appear to increase risk of infection of hospital workers 10 fold (more face touching and cloth has very poor filtration). Masks usage to prevent getting infections from a sick family member don't seem to be effective. The big question - which we don't know - is cloth sufficient to reduce spread of virus from asymptomatic individuals; and are people likely to get virus on their mask and then infect themselves due to increased face touching; and which of those is the more likely scenario.
https://arstechnica.com/science/2020/04/should-you-wear-a-face-mask-heres-all-the-data-we-haveAs to countries with high success controlling the virus also being countries that were wearing masks - Singapore didn't have mask wearing till the other day. Thus most of the success is probably the other measures - their rapid response track and trace and testing.
