Why was it necessary for marshall to take the trouble to explain why navigation should considered as part of commerce?

It was necessary for Marshall to explain why navigation should be considered as part of commerce in order to establish the broad scope of Congress's power under the Commerce Clause. By including navigation as part of commerce, Marshall expanded the definition of commerce to encompass all aspects of commercial activity, including transportation. This interpretation allowed Congress to regulate navigation as a means to facilitate and regulate interstate commerce effectively.