Probably because he couldn’t sift through so much material on his own and he was never going to get another chance to go back and retrieve important corroborating evidence after he took off. If you’re only going to get one go of exposing something, take it all so you’ll definitely have whatever you’ll need, then get help figuring out which is crucial and which is not.
Think of it like planning to take a very, very long trip. If you could pack your whole house up, you would, why wouldn’t you? Who knows what you’ll regret not taking? And in terms of needing to prove every small thing Snowden was planning to claim, he was going to need as much evidence as possible.
And as for the info being “worthless”, according to our own NSA they weren’t using most of it anyway, right? Far as we know, they raked in a lot of funding to make it all happen and never used it to much proper end that older, cheaper, legal methods of espionage don’t already cover.
