Why need "all" in this sentence? Does it give any extra meaning or they're the same?
It doesn't seem that difficult.
It doesn't seem all that difficult.

Question

Why need "all" in this sentence?  Does it give any extra meaning or they're the same?
It doesn't seem that difficult.
It doesn't seem all that difficult.

Answer

"All that" is an adverb meaning "very". In the first sentence, "that" is an adverb meaning "particularly". The expressions are not exactly the same, but I'm not sure the difference is worth talking about.

Answer

They mean the same thing, adding "all" just gives it a little more emphasis I guess. No difference, really.