How do gps and imu work together, and why do need interpolation ?

In function `PrepareLocalizationMsg()`, why do need interpolation?
```c++
  CorrectedImu imu_msg;
  FindMatchingIMU(gps_time_stamp, &imu_msg);
  ComposeLocalizationMsg(gps_msg, imu_msg, localization);
```

Find nearest IMU and the put into localization. IMU adds several types `linear acceleration` `angular velocity` `euler angle`