This gives you time to learn what's going to happen in your incubator, and allows you to make any necessary adjustments before setting your eggs. A sure-fire way to ruin hatching eggs is to put them in the incubator without having it properly adjusted. This is plain and simple, yet the MOST important part of hatching. You can sneak by with humidity numbers that aren't very accurate, but the combination of poor humidity and temperature will definitely cause problems at hatch time. I'm willing to bet that it isn't.
Thermometers go bad.
Keeping the temperature accurate can be a struggle, even with very good thermometers. I threw away 2 thermometers this year that I had considered reliable. A nice part about running a big incubator over an extended period is that you can tweak the temperature regardless of what thermometers tell you. After the first hatch, you can raise or lower the temperature by what the hatch tells you. If they hatched early the temperature needs to be lowered.