In the game there are packages spread all over the map and I wonder why I should find them (except for the small amount of money). I'm only talking about the packages marked by a point on the map, those that the factions are looking for. Thanks for helping, maybe someone got them all?
Resource items include vehicle and weapon upgrades (Vehicle Parts and Weapon Parts), upgrades for Rico's maximum health (Armor Parts, health is upgraded after 5 are collected), and cash bundles of $2500 each (Cash Stashes). There are more than enough weapon and vehicle parts to fully upgrade all 22 black market items to maximum, with plenty of excess parts that you will never be able to use.
Faction items give you money (to buy equipment and vehicles at the Black Market) and progress the game by giving Chaos points.
Aside from the regular rewards you get for each one, there is no special unlockable content for collecting all of them as far as I can tell.
However, there are achievements tied to the collectible items (From GameFAQs):
Faction Benefactor - Collect 150 faction items.
Finders Keepers - Collect 100 resource items.
Leaving No Rock Unturned - Collect 1000 resource items.
These acheivements are the same for PC, PS3, and Xbox 360.
EDIT: This forum thread also suggests there is no particular reward, but I'm not sure how definitive that information is.
Having done this on the PC version, the reward is... nothing. Well, aside from the cash, parts, health and so on, the related achievements listed in Calvin's answer, the satisfaction of doing it, and a de-cluttering of the Just Cause 2 Map Viewer (which I heartily recommend if you're going to try and do this on PC).
I'd suggest getting the achievements/health levels/upgrades you want and then not worrying about them. Go and blow stuff up instead!