Marshall ruled that when an issue such as navigation crosses the state border, it also becomes subject of Congress because of the trade clause.
Once the state border is crossed, Congress has full power.
It was necessary for Marshall to take the trouble to explain why navigation should be considered as part of commerce so that people understand the laws. To help people understand why this law was necessary, he had to explain why it was constitutional.
Because if a trucking service moves between San Francisco, California and Portland, Oregon, it could be subject to regulations from both the state and the federal government.
So the truck service would have to respect the laws of California and Oregon, because the states must respect the laws of other states, as it is included in the Constitution.