Well Bella needs to get a Birth certificate and some stuff for Jake and Nessie . Jasper left to go find some of his বন্ধু so they could help in the ''fight'' And Alice went to so Aro wouldn't take her back to Volterra. They really need Alice's gift. I hope i answered this how u wanted it .
Alice and Jasper leaves because Alice want to see if she can find any one like Renesmee. To প্রদর্শনী The Volturi that Renesmme will not become a treat. She sends Bella to J. Jenkes, so Bella can protect Renesmee and Jacob if the metting with The Volturi ends badly.
They left because Jasper went to get his old বন্ধু to help persuade the volturi, and Alice went with him so they could also find Nahuel(who is just like Nessie). Alice makes Bella go see J. Jenks so she can get birth certificates and such so that if things went wrong Nessie and Jake could escape....hope i helped any...lol