Why does the speaker say "seem to disappear" not just "disappear"?

Native speaker’s answer
Rebecca
The speaker says, "seem to disappear" since the moon does not literally disappear. The word "seem" means that it appears as if it did disappear. However, it's still there even though you can't see it. Ex: It seemed like everyone knew about the party already, except for me. => It appeared as if everyone knew, but it's not certain. Ex: The cat seemed to disappear into its bed. But the bed was the same colour as the cat's fur.