Need to run a certain operation on a bunch of lines? E.g. add some text after a certain column or to the end of every line in a buffer? With keyboard macros you can perform your line operations "live" for a line, while recording them, and then replay them for the other lines.
How to record a macro: position yourself to the beginning of line, press C-x ( to start recording keyboard macro, then do some stuff such as C-e to jump to end of line to write some text, then press C-a C-n to jump to start of the next line, then C-x ) to end macro recording.
How to replay a macro: C-x e to replay it once, then keep pressing e to replay it for other lines. Or, to replay it for the following 1234 lines, do C-u 1234 C-x e. Or, to replay macro to all lines in a region, do C-x C-k r.
One can combine macros with register counters in order to rapidly help Bart Simpson:
C-u 1 C-x r n a ;; store number 1 in register `a' C-x ( ;; start recording macro C-x r i a ;; insert contents of register `a' C-f . I will not waste chalk. RET ;; enter our text C-x r + a ;; increment register `a' C-x ) ;; end recording macro C-u 7 C-x e ;; apply macro 7 times
which will produce this output:
1. I will not waste chalk. 2. I will not waste chalk. 3. I will not waste chalk. 4. I will not waste chalk. 5. I will not waste chalk. 6. I will not waste chalk. 7. I will not waste chalk. 8. I will not waste chalk.