What reasons why people File a Chapter 13 over a Chapter 7 bankruptcy?

After reading hundreds of posts on here, the one question I constantly ask myself after reading some peoples posts is "Why didnt they just file Chapter 7"?

What are the benefits and affects of filing Chapter 13 over a Chapter 7?

Also a HUGE question: What happens when you default on a Chapter 13? Seizure of assets and sued? You cant file your way out of that for protection...