So yes, it was necessary to keep people away from the debris field, and to haul everything away to another base. But for the remnants of the highly classified Project Mogul, during the uncertain days of the early Cold War, the secrecy oath made sense. Or so went the local reasoning. And some of those people were probably local military men, even officers, who were kept in the dark about Mogul. So of course scuttlebutt abounded, rumors right there on the base that a saucer had crashed, that aliens had been seen.
Military men love rumors as much as the next guy.
Wild gossip and rumors make a dull routine less dull. The Soviets would then shrug that whole Roswell UFO tale off as hysteria, fallout from the Kenneth Arnold sighting. So, yes, let the myth grow, maybe even encourage it a little. They were well covered. Nowadays, some investigators feel that Mogul can't explain the crash because the alleged dates of Mogul launches don't properly coincide with the crash.