What is "Resource Mangager" and can I add it later?

What is the "Resource Manager" upgrade? I know the description says downloads and article-like content but I am not sure what that means. Does that mean users normally can't upload attachments without it or is it something different?

Likewise if I feel I don't need it or "Enhanced Search" right when I start my forum are these upgrades something I can add later?
Thanks for the information @Brogan I will probably just purchase the base licence for now.

Its nice to know that I am free to upgrade at my own leisure after I have a good gauge of XenForo and whether my community will need these features or not.