To execute commands at a Unix prompt as sub-processes e.g. echo `date` will run the date command and echo the output.
You can always use the (preferred since POSIX adopted the Korn shell-isms) $(date) vs `date`. But it would also mess with m4 open-close `' style quoting.
It's funny how characters that were not important to English typists were defined to become important (~, `, @, *, etc.) just because they were on the keyboard, so got used for something.