Minecraft wiki entry for Defeating_a_monument, associated text: To get the 8 gold blocks in the core.
To get sponges
To obtain sea lanterns, and prismarine, which can otherwise be obtained only from underwater ruins.
To set up a guardian farm, which allows you to get even more prismarine, sea lanterns, and fish.
For the experience you get when killing guardians.
To establish an underwater home (or even an underwater village).
For playing mini games with your friends.
For the challenge.