It's basically there to make Ruby an equivalent to AWK. It only makes sense if you are looping over the lines with -n. The BEGIN block runs at the start of the run, and the END block at the end. They are useful for things like initializing a sum variable to zero at the start and printing it out at the end.

It's from awk: the BEGIN and END patterns match the start and end of a file/stream being processed, while normal patterns are checked on each line (and no pattern matches every line).

This is very useful to script a setup -> for reach line -> summary of a file e.g. initialise a map in BEGIN, fill the map for each line, and print the map in END.

What really baked my noodle is that there is another "end" called __END__ used like this

  $ cat data.rb 
  DATA.each_line { |ln|puts ln }

  __END__
  fish
  cat
  dog

  $ ruby data.rb 
  fish
  cat
  dog