C3PX said:
The point was this giant war machine was coming to destroy the planet they were hiding on, which would have been a massive loss to the Rebel cause. And once the Empire had a weapon of mass destruction with the ability to destroy entire systems, their would be no stopping them. The glimmer of hope were the Death Star plans hidden in Artoo. By examining these plans, they discovered that the weak spot, this was their only hope.

The Battle of Yavin was a final stand, they could either retreat by evacuating the planet, or they could fight back and try to end this huge threat on the galaxy. They choose to do the later. They sent in all their fighters and pilots, on what was essentially a suicide mission. They were throwing everything they had at the Death Star in desperate hopes of being able to get some torpedoes into that exhaust port. Of course the pilots were suppose to protect one another and shoot down as many Ties and they could. The Y-Wings, which are bombers, were the ones intended to give the blow to the Death Star, and the X-Wings were really their to give the bombers cover fire, but in the end the job was left to the last fighter left standing, Luke and his X-Wing.

i just don't get why the xwings didn't fire at the ties chasing the ywings and the other xwings. Han did it to save luke's crack!