Good Morning, is anyone able to help me with understanding why it would benefit an author to copyright and trademark a character from a fictional series of novels?

This is my understanding of what they are asking in the question below? I am looking to discus the reasoning behind this choice.

Many novelists, particularly those involved in the writing of genre fiction, have one principal character that is featured in most of their books. The sea tales of Clive Cussler, for example, all feature the adventures of fictional marine engineer Dirk Pitt. Cussler's novels are all copyrighted, but the name Dirk Pitt is registered as a trademark and appears with the TM logo on the back of the books. Why is it in the interest of the author to trademark as well as to copyright his fictional creation?

https://www.copyright.gov/

https://www.uspto.gov/