Mostly to fight the Soviets which had occupied Afghanistan. He used his personal connections and wealth to supply resistance against the Soviets there and had even gone on to fight with them. He was also disgusted by Saudi Arabia's apparent reliance on the West.
Bin Laden went to Afghanistan out of religious purposes, to aid the resistance of the mujahideen, or holy "strugglers," against the Soviet-supported government.
He wanted to help other Muslims fight against the Soviet occupation.
