"The moon is round; the mooncake is sweet."
It sounds like something a kid would say, common sayings which also apply during any full moon would be:
The latter two are from poems.
In most cases you can't have an adjective directly follow a noun (with only a few special exceptions). You need to have a "connecting" word sandwiched between them - by default, 很.
This is also why the "very" is optional in translation, e.g. this sentence can be "the moon is very round", with the 很 actually meaning "very", or "the moon is round", where the 很 is just connecting the subject and the adjective, kinda like the "is" in English.