Sanji's role among the Straw Hat Pirates is an important one. He's the chef and nutrition expert, making sure that the rest of the crew eats healthily during their travels. When Luffy first started out on his journey, he knew he wanted a chef but... well, let's face it; Luffy's concerns weren't really with being healthy. He just wanted to eat. ^^; It was after Yosaku fell ill to scurvy that the Straw Hat Pirates decided that they would enlist a chef as their first priority.

Luffy found Sanji at Baratie and with some luck and determination, got Sanji to join him. Many times has Sanji saved their bacon by providing healthy choices in food and drink. It's his duty not only to prepare all of the meals but to go grocery shopping when the ship lands at a town. Sanji generally takes his shopping very seriously, picking the freshest of foods and storing them properly so that they won't spoil too quickly (then again, nothing gets a chance to spoil while Luffy is around). He also gets pretty pissed off when Luffy and the others sneak food from his pantry when he's not around. After all, if anyone knows the horror of starving at sea, it's Sanji! He's definitely a good choice for chef aboard the Going Merry.

Aside from cooking, Sanji is also one of the three powerhouses aboard the ship. He usually finds himself defending the Going Merry and its crew on several occasions, being the strongest aboard the ship along with Zoro and Luffy. If it wasn't for him, I think the Straw Hat Pirates would find it immensely difficult to travel the Grandline, even with Zoro and Luffy there.

Without question, Sanji is certainly one of the most important members of the crew, alongside Nami and Chopper. He keeps them all healthy and makes sure that no one goes hungry. I think without him, Chopper would probably be a lot more busy. ^^;