The debugger may well be the most sophisticated bit of software SAP has ever produced. Like every bit of SAP, the debugger has undergone a lot of upgrades. From version 6.40 there is a Classidc debugger and a New Abap debugger. The main difference is the way the debugger shows: the New Abap debugger presents itself in a new session, which allows more insight with less impact on the program that is being debugged. These articles are all about the New Abap Debugger.
Why debug ? Without the debugger you will be guessing what is happening in your program. The debugger will explain in crystal clear facts what happens and when it happens. Debugging has a very high "Ah, right" factor.
Once the debugger is invoked, the user has access to an elaborate set of tools with which the runtime "live" information can be viewed (and even changed). Coding can be stepped through line by line (F5), to a given line (containing a break-point or watch-point, F8), skipping routines (execute the routine, method or function module as if it was a single step F6) or stepping out of the routine (method of function module) you just stepped into (F7). Watch your coding come to life in the debugger.