"The tall man is outside."
Translation:A magas férfi kint van.
I believe it's the difference between "The tall man" and "The man is tall"
"Van" can be left out when someone is performing an action in the 3rd person singular: "He is speaking Hungarian" = "Ő magyarul beszél" In this case, you don't have to say "van" (is), like you would have to in English (in fact saying "van" in this sentence would be incorrect)
But in the sentence "A magas férfi kint van.", "van" has a different meaning. It means "there is". Another example would be: "Van két almám" = "I have 2 apples" (a more literal translation would be "There exists 2 apples which are mine"... where "van" means "there is / exists" and "almám" means my apple")
So you see, "van" can also be used as "there is". And in this case, the sentence wouldn't make sense without it.
As far as i know van can be left out when it refers to adjectives. In this case it refers not to how is the noun, but where is the noun, thus van is needed.