The short answer is biochemical reactions involving products and intermediate products from other biochemical reactions. For review, here is a good article to get you started:
To answer the second question, amino acid synthesis is essential for life because amino acids are the building blocks of proteins which sustain life.
I know this is a short answer. If you want me to go into more detail, please feel free to contact me. There is so many reactions to talk about and I love explaining this to students.