REGEX can be used for a ton of things. Some common uses are finding parts of a string, substrings, remove all non characters from a string, removing all characters, removing all numbers, getting strings between strings etc etc etc. It is a insanely handy tool.
Say you had a file of phone numbers and the file was:
NUMBER: 250 -- 305 1982
250 309 3198
NU 250 MBE -- 393 --R 1982
Its all messed up with some terrible formatting etc right? Well Regex could strip all non number characters out and provide a clean file full of nothing but phone numbers.