Because it protected their basic liberties, but most importantly, it guaranteed the constitution would be approved.
The amount of territory the US currently has would have been completely different if the national government could only legislate based on what is written in the constitution.
A perfect example of this is the power to purchase land/territories from other countries. This was first done by Thomas Jefferson with the Louisiana Purchase of 1803. The US Constitution does not say that the president has the power to buy land from other countries. However, Jefferson said that this power was "implied" by the Constitution.
This idea of an implied power greatly changed the presidency/national government. Several presidents after Jefferson would follow his lead by making deals with countries for territory.
If it wasn't for this concept of implied powers, the US may not have grown to the 50 states we know it as today.
The Duke of York owned Delaware after the British/English took it
Zebulon Pike explored the Great Plains, the Rocky Mountains, New Mexico, and Texas
