What is the law of atonement when it comes to Jesus Christ and God's plan for the salvation of humanity? Why was it necessary? Couldn't God have planned a different way to accomplish His purposes?

Here's a starting point for more study on this topic from 2 Nephi chapter 2 https://www.churchofjesuschrist.org/study/scriptures/bofm/2-ne/2?lang=eng&id=p7#p7