关于英语的一个疑问：“"and"的用法”

关于英语的一个疑问：“"and"的用法”
在《新编大学英语》的教材上,Unit2-Passage Reading "Special Delivery"中有一个句子：“My brother's flight arrived and,after getting his suitcases,we headed for my car in the parking lot”而在Unit-5Passage2 "Ready for Danger"中有这样一个句子：“But some animals do not produce so many young ,and so more of them have to survive.”两个句子中“and”的意思没有变化,但位置变化.这样做有什么好处,或者有其他原因?
But some animals do not produce so many young,and so more of them have to survive.如果也把这句中的and放到上一分句中会有影响吗?