Without calibration, the battery percentage reading will be incorrect, and your device may behave oddly—shutting down suddenly even though the new battery “reads” half charged, or working for hours when the battery reads nearly dead.
The fundamental problem is that there’s no reliable way to know exactly how much energy a battery holds at any given moment. (It’s an electrochemical storage system that is always changing and decaying, and never behaves exactly the same way from one charge to the next.) About the only reliable way to gauge it is to fully charge the battery, then fully discharge it and measure the difference (a.k.a. coulomb counting). Obviously, we can't do that every time we want to check the battery level, so we have to use indirect methods—storing all kinds of usage data and using that to calculate an estimated % state of charge from moment to moment. Over time, that calculation tends to drift and become less accurate. And on a brand-new battery, there’s not really any good data to work with, so the model will be way off. Calibration helps keep estimates accurate by setting new “full charge” and “full discharge” anchors in the battery management system so it doesn’t have to guess.