[atroyn]

This is a complex question. As you pointed out, having a high fidelity map helps to identify the part of the environment that change, creating 'deltas'.

Another reason is that autonomous vehicles take data from a wide variety of sensors, including cameras, RADAR, LIDAR and others.

None of these modalities is perfectly, and there is always ambiguity and drift. High fidelity, high resolution maps provide a strong prior that helps to resolve these issues.

Besides the sensing issues, high fidelity maps also provide priors with respect to planning and prediction of the behaviors of other road users. A busy intersection should be approaches differently to a small backroad.

In the end, autonomy needs both real-time processing and high fidelity maps to perform effectively.