IMUs (Inertial Measurement Units) consist of gyro's and accelerometers that measure change in movement over very small increments of time. The sensors, particularly the gyro's, drift over time and become very inaccurate.
By applying a synchronized, high precision GNSS trajectory, the drifts in the sensors can be corrected with a process called Kalman Filtering.
With a highly accuracy sensor trajectory and orientation, data produced by hardware such as Lidar scanners becomes inherently more accurate and will achieve better results with less dependency on ground control.