More than 220 million people speak French on all the five continents. French is a major language of international communication. It is the second most widely learned language after English and the sixth most widely spoken language in the world. French is also the second most widely taught language after English, and is taught on every continent. French test will reward you some points in your express entry profile as first or second language. In addition to those points there is a nomination stream in Ontario for French speaking workers. If you speak French and English you may qualify.
Writing a French exam can help you claim additional points for second language and overall your ranking can increase. For Quebec province too French language exam will be beneficial. Quebec has a separate immigration program than that of Canada. It can be real quick to become a Permanent Resident if you decide to live in Quebec for a while as there is a fast track program for francophone. Once you become a PR, you are free to move to any part of Canada.
It also makes entry into other countries easy and you have an edge over the rest professionally.