Say you have a text file and need to extract all its email addresses, comma-separated.
I'm more comfortable solving a problem like that in Ruby than in pure Vim – especially if it gets more complicated, perhaps with sorting and grouping.
When it's a quick-and-dirty one-off thing that I don't need to save, I often write that Ruby in the data file itself, instead of bothering to create separate files. It's a convenient trick.
For example, if I had the file
foo firstname.lastname@example.org bar email@example.com …more text here…
and wanted to extract comma-separated emails, I would just add something like
foo firstname.lastname@example.org bar email@example.com
…more text here…
You could then make Vim run the script with
:w ! ruby.
If you want the output in a file, do
:w ! ruby > emails.csv.
Or if you want the output to go into a buffer, try my vim-ruby-runner.
If you didn't know, anything following
__END__ in a Ruby file becomes a kind of virtual file that
DATA.read reads from.