The purpose of the words "hour" and "spring" is <em>to remind the readers that they must take action immediately to succeed against the British.</em>
These are Thomas Paine's words and they mean that he knows that the army is tired and that the men need rest but their safety depends on them. They have a lot to lose if they doubt so they have no time to think (hour) and time has come to drive off the invaders. They have to take action immediately (spring).
Poetry was read and studied by higher classes. When middle classes began getting educated novels started becoming the favorite literature of the masses.
