As the battle progressed it became clear to Grant that a fall back line was needed. He gave this task to his chief of artillery, Colonel Webster. This final line utilized whatever available artillery was nearby, including some siege guns that were intended to be used at Corinth. As units retreated from other portions of the battlefield they further filled in the last line. By the time the Confederates attacked it late in the day it was very strong and they were unable to breach the line.
Grant could have decided that a fall back line was not necessary. He could have decided to send every available unit back to the front. He could have just relied on his commanders to rally their men when they fell back to the landing.
If the line had not been made it would have probably had severe repercussions when the Confederates made their attack on the Dill Branch line. By giving retreating units a safe place to halt, it was easier to rally those men. It is possible that they could have continued north until they felt safe before stopping. A reserve line studded with cannon helped them feel safe where Grant needed them most.
This was an important decision but not a critical one because it seems obvious to create a fall back line. Any competent commander should have built a last line, especially in a situation where there were no more fallback points. North of Pittsburg Landing there are not any other obvious fallback positions, if needed the Union would have found something but probably nothing as good as the line along Dill Branch. Grant made a good decision but because of his situation, terrain and otherwise, it was not critical because it was so obvious. If he had made the opposite decision that would have been a critical decision, and a mistake.