If a piece is written in e.g. even eight notes, but you want to indicate that the rhythm of the even eighths should be played e.g. with a swing rhythm, you can use this. This can be used and tweaked for various rhythm styles/changes.
This snippet is obsolete with LilyPond 2.23.x: use the new built-in \rhythm markup command, e.g.,
\tempo \markup { Swing \rhythm { 8[ 8] } = \rhythm { \tuplet 3/2 { 8( 8) 8 } } }