Why was it important that achilles gave priams son back?

Priam pulled on Achilles' heartstrings, so to speak. They had a little 'bonding time' when Priam showed up for Hector's body. At first, Achilles refused Priam's request, but soon Priam mentioned Achilles' own father, which ended up leaving them both in tears. The affects of war upon a family is severe, as seen in this scene, when two enemies morn what they have lost.

http://www.gradesaver.com/iliad/study-guide/sectio...

So he could give his son the proper funeral rites.

This was believed to be important for having a good afterlife (getting to their idea of "Heaven")

it was a Greek custom. You have to do the whole burial process so that Hector's soul can go into the next life. this relates to modern times. Who would settle to have the memory of their loved one not simply tarnished, but utterly humiliated? Despite Hector being dead, he still deserved to be handled with dignity, especially in the eyes of his own father.

en.wikipedia.org/wiki/Priam_of_Troy

wiki.answers.com/Q/Why_does_Priam_go_to_​Achilles_camp

Without a proper burial, Hector was doomed to roam the earth as a ghost forever. Priam had to bury him to forestall this fate, according to custom. Priam, at tremendous risk to himself, disguised himself as an old beggar, and went into the Greek camp, to claim Hector's body, so he could bury him with the honour he deserved. And in doing so, shamed Achilles, who was a bit of a glory hunting psycho killer. Hector fought honourably, for the defence of his City, while the Greeks were really just there for the plunder, and because they had been bullied and cajoled into it, by Agamemnon.