I often use grep and ag to search for patterns in a group or directory
of files. Generally I am interested in looking at the matching lines
themselves. However, sometimes I just want to know the set of files that
have matches. Both grep and ag can be told to output nothing more than
the names of the files with matches when given the -l flag.
This can come in particularly handy if you just want a list of files that can be piped (or copied) for use with another command. This eliminates all the extra noise.
h/t Dillon Hafer