You may want to know whether is single or multiple fracture. If it is a serious break involving more than one rib, you could have injuries to internal organs as well. Plus its free (or not - I have sometimes been charged for x-rays and CAT scans usually like ~$20, sometimes not.) Plus you get cool pictures of your insides to post on the interwebz.
If nothing else, to monitor the progress of the healing. If for some reason a bone fracture is not healing/bonding then you may need a bone graft depending on the case. But hey, if you want to go it with not a clue with what is going on, only an assumption, buy all means giver.
I skipped the x-rays for what felt like a single cracked rib and took it easy until it felt better. If the pain was more severe an x-ray would've been appropriate. Ultrasound to check for internal organs makes sense, but it wasn't that bad a crash.
For the 8-pack there was no choice. The x-ray I got was good for bragging rights but there was no followup x-rays to monitor nor any treatment.
Come to think of it, the x-rays got me evil but effective painkillers! :high: