The road which the sentinel guards is important because it stays next the enemy camp, in which the troops allied with this sentinel will do a surprise attack the next day, being the sentinel a key component of the strategy.
This affirmation is supported by the following excerpt:
"(...)concealed in the forest at the bottom of that military rat-trap, in which half a hundred men in possession of the exits might have starved an army to submission, lay five regiments of Federal infantry. They had marched all the previous day and night and were resting. At nightfall they would take to the road again, climb to the place where their unfaithful sentinel now slept, and descending the other slope of the ridge fall upon a camp of the enemy at about midnight. Their hope was to surprise it, for the road led to the rear of it. In case of failure, their position would be perilous in the extreme; and fall they surely would should accident or vigilance apprise the enemy of the movement"