Umm...

  * Tracing code execution paths to load the code into your mind
  * Modifying state inline so you get a different path without re-running
  * Modifying state like FirstName to trace just how the bugger gets into the output
  * Some sophisticated debuggers like java can allow you change code, then hot recompile and deploy code LIVE.
  * Suspend threads when you reach a breakpoint so you can dig around at all the multi-threaded state at that point
     - Although multi-threaded debugging is where printfs shine

All of that is much more difficult with just printfs.