When I was on like level 20 in single player i used to find these guys everywhere, and i thought they were only useful for a handful of chitin. Now I’m on level 88 and i actually NEED black and silicia pearls and oil I haven’t seen one in months.
These are a good source of chitin for beginners, since the first set of armor you're probably going to make once you get a smithy is chitin armor, and trilobites are the first source of chitin you will see on the island.
Fun fact: The trilobites appeared during the Cambrian explosion and was one of the most successful lifeforms on the planet. Nature had to try and wipe them out about 3 times and it took the most deadly mass extinction in the history of our planet to wipe them all out. The Permian Extinction also known as "The Great Dying"