If Sheeta immediately surrendered to Dola's gang at the beginning of the movie, she would never meet Pazu, but Muska would NEVER find Laputa to use it for evil, and it would never be destroyed.
At first it seemed strange that Sheeta's necklace would allow her to float down to the ground, even though she was unconscious and clearly not controlling it. But when it's revealed that the necklace was worn by Laputian royalty it suddenly makes sense. The necklace was meant to act as a safety net for people who accidentally fell off of Laputa!
Muska's eventual fate. Imagine falling thousands and thousands of feet to your death— while completely blind.
At the film's climax, after Muska has taken control of Laputa, he tells a captive Sheeta, "You and I will be spending quite a lot of time together." Keep in mind that these two - a grown man and a young girl - are the last of Laputa's royal family, very likely the last two Laputians alive. If Muska's aim is to bring Laputa back to power, that likely includes repopulating its people.