What is the effect of donating to Allied Supply Crates?

Aside from the XP bonuses, the actual direct effect of donating to the allies isn't that clear. I wouldn't bother spending an excess amount of time collecting materials. Just throw whatever extras you have to them for the bonus xp, but going out of your way is unnecessary.

This page details a little more info on which items are specifically needed for each ally.

Donating Elfroots is the most efficient way to get XP because you can buy them so cheaply from the elvish camp. See the Dragon Age Wikia Page on Allied Supply Crates for more info.

I can't answer your question about what the benefit is, but I can tell you that I didn't donate a thing to any of the factions and still finished it with relatively no bother. So you can conclude it doesn't play much of a role.