Those commands may look useless at first sight, but combined can create powerful routines. As an example, my routine to refactor non-typesafe codebases.

- Find the pattern that you want to replace (the last dot is for current dir)

:grep -r 'pattern' .

- Open the quickfix window and check if matches are correct

:copen

- Find and replace the entries of the quickfix list

:cdo s/pattern/replacement/ | update