No one likes debugging code when it breaks and you can't figure out why it's broken. That piece of code might have been hard enough to write in the first place, or maybe it's a snippet that "should work" from a coworker, or maybe the documentation is missing, or maybe.... But what if you've checked, double and triple checked, and it's still not your code, it's something else in the system – code you're calling out to, or something that is calling your code – what do you do then?
Enter the backtrace, (also known as stack trace, or just trace). A powerful weapon in any software developers toolkit, it is more and more useful as the software you develop grows in complexity (Drupal 8 anyone?)