Jesus challenged the disciples to give the 5,000 people food because:

1.  What is impossible for man is not impossible for God.  Through faith, food can be multipled and the hungry can be fed.

2.  Without feeding them, they would have either been sent away to buy food or would have been hungry enough to leave and buy food.  Salvation is worth more than food and, like the passage says, Jesus felt compassion for them.

3.  To show that God will provide and faith in Him is rewarded (the disciples took up baskets full of leftovers).