Leia trying to recue Han and failing before Luke came was very important, just think if Leia's rescue had come after Luke was already there, then you can almost be sure the plan would have worked out fine. Leia would not have been captured, and the whole sarlacc sequence would have never happened. Luke's hand would not have been shot and he wouldn't have have worn that cool black glove on his hand for the rest of the movie. And not to mention million and millions of fans would have missed out on Carrie in the gold bikini.