For the jangerberries uses :
1- they can be eaten , restores 1 prayer point, 2 hp, 2 attack, but reduces defense (1 if i remember right).
2- They are a necessary ingredient during the watchtower quest.
and regarding spider eggs, they got a huge whack since the release of the summoning, as one of the monsters you summon (spiders) can spawn them, fast, and reliably.