Why is the cracking of petroleum fractions so important?
Tar isn't worth much, gasoline is, so if you can turn tarry substances into lighter compounds, you can make (more) money. Which you do by cracking; breaking up those BIG molecules into smaller molecules. Side note; car tires can be roughly 50 % carbon black (of different types) by weight; carbon black is produced by thermal cracking of heavy oil fractions. That method pre-dates the cracking described above by decades.