If you know exactly which statement caused an error, the use of a single breakpoint may be sufficient. More often, however, you can only guess the general vicinity of the statement that caused the error. Setting a breakpoint helps you get to that general area; when you get there, you can step through your code and execute it line by line to see the effect of each statement.
You can also step through each line of code in a called procedure (a procedure the current procedure calls) or step over the called procedure code.