The confession scene was especially poignant because the whole theme of GFIII is Michael trying to redeem himself. He wants outof the mob and into legitimate business, he wants to reconcile with Kay and his children (or at least end the estrngement) and he wants to go into his old age peacefully.

BTW the the coinfession scene the priest does a lot more than say go and sin no more. He admoinishes Michael and he says "Your sins are terrible" and he says that Michael can yet be redeemed, but that he probably won't be becuase he (Michael doesnt really have any faith.

All of this ties in nicely with the price Michael pays for redemption -- the death of his daughter.

For all the trashing of GF III this symbolism alone makes the film worthwhile.