Just google - there are many articles about. There is some spin to say the he really believed that masks were not necessary. Some article agree some don't. Clearly, he did it purposefully based upon interviews he gave.
Here is a piece of an article from Slate:
In March 2020, as the pandemic began, Anthony Fauci, the chief medical adviser to the president of the United States,
explained in a 60 Minutes interview that he felt community use of masks was unnecessary. A few months later,
he argued that his statements were not meant to imply that he felt the data to justify the use of cloth masks was insufficient. Rather, he said, had he endorsed mask wearing (of any kind), mass panic would ensue and lead to a surgical and N95 mask shortage among health care workers, who needed the masks more.