Note: This question is not about whether or not an alien craft did crash at Roswell, but the fact that the on July 8 1947 an actual spokesman for the base told a newspaper that an alien ship had crashed, and showed debris.
It just seems like the absolute worst thing you could do, regardless of whether it was a secret project or an actual alien ship, as its a great way to attract attention to something you'd want kept secret (even if it actually didn't get much attention for 30 years).
So why did they do it? Misinformation? Is there precedence for leaking an unbelievable story? Just a bad idea? Or was the spokesman just a guy trying to get attention?
According to the US government narrative, a rancher ran across some classified programs. And it all got out of hand because a local US Air Force press officer saw some stuff he shouldn't have and reported it as flying saucer debris.
There are some pretty questionable issues with that story, and 60 years of a theoretical ecosystem centered around Roswell and UFOs and aliens, but that was the official line.