Here is James Tanner’s answer (from “What is Metadata?”) on FamilySearch to the why metadata question:

OK, so why add metadata? This is also an easy answer. You add metadata so you can find photos on your computer and remember the information about who, what, when and where. Once the metadata is added to your photos, you can use your computer’s search function to find the photos by searching on any of the words or names you have added to the photos and stored with them in their file. How will you know if your program is storing the information with the original file or in a separate file? You might need to search in your program’s preferences or options to find a reference to adding data to your photographs.

Adding metadata to your photograph not only helps you but it helps anyone you might share that photo with, if they know to look for it. The metadata stays with the photo so anyone who has the photo can get access to the information and know everything you know about that photo. Adding metadata takes time but when it comes to family photos and documents, it is the who, what, when, where etc. that makes the photo have value.

Metadata can also be added to other types of files other than photographs. Denis Barrett Olson wrote an article about using metadata to establish provenance. There are lots of ways to use metadata to organize files and make sure that important information about a file stays with that file.

Now we know why we want to use metadata, the challenging part is actually taking the time to add it to our files.