Who says that you need to be able to speak coherently to be a valuable member of the crew? Chewbacca may only be capable of speaking Shyriiwook, but he's just about the most capable and loyal co-pilot you're likely to find anywhere in Star Wars. Many years before A New Hope, Han Solo refused to follow Imperial orders and kill Chewbacca, so Chewie swore to protect him for life as repayment. Thus the greatest bromance in a galaxy far far away began. There's clear camaraderie, affection, and respect between the two characters, and it's obvious that Chewie would lay down his life for Han and vice versa. It's not hard to see why he's such an iconic character: he's in one of the most famous film franchises ever, and he's basically Bigfoot with a crossbow.