Ever tried laying a long straight run freehand? I used to think a level was overkill for small jobs, just like you said. Then I helped a buddy with a short garden wall and we skipped it, thinking our eyes were good enough. Two weeks later you could see a slight but obvious slope in the middle when it rained. That tiny bit off at each brick adds up way faster than you'd guess. Now I just suck it up and use the string for anything longer than a few feet, it saves the headache later.
Why do we always trust our gut over a cheap piece of string? I skipped it on a small patio job and thought I nailed it until the next morning. Sunlight showed every wobble, and I had to pull up half the bricks.