Some maps and mods can't be found on the internet anymore, but they can be found on some WoP servers. (For example, ModKuh or Beryllium)
In such situations, WoP has a useful "Autodownload" feature, which automatically downloads every .pk3 file that the server has but the connecting player does not (that includes skins, music packs, mods and the current map). If Autodownload is turned off and one tries to connect to, let's say, a server with a map that the player does not know, they won't be able to connect.
How to turn Autodownload on? Edit
Autodownload was moved in 1.5 which confused some people. Here's how to activate it on both major versions:
Go to Setup->Options. The switch is set to OFF by default, and it's on the bottom of the list labeled as "Automatic Downloading"
Go to Setup->System->Network. The switch is set to OFF by default, and it's on the bottom of the list labeled as "Auto Download"