In other words, they didn't want the states to be forced to defend themselves individually. They realized that we all share common borders, and we should all share in our common defense.
States have the power to make, collect, and spend their own taxes.
Each state is unique, with its laws made by the legislatures and governors. The state pays for welfare and special services programs. The Federal Government's responsibility has grown since then. And you're right, states sometimes impose taxes on these same things, too.