What is the purpose of Docker.raw file on Mac OS Catalina?

After having pruned the unused docker objects ( https://docs.docker.com/config/pruning/ ) in order to reduce the size of the file Docker.raw, open the Docker Desktop preferences, and then on the Disk tab:

You can reduced file to less than 64GB. it's telling you how much is currently used the current data. Also Resizing to a smaller size will delete the disk image; all Docker images / containers will be lost. Other settings will be preserved.

The answer to your question is in the Docker documentation:

Disk utilization in Docker for Mac

Docker Desktop stores Linux containers and images in a single, large “disk image” file in the Mac filesystem. This is different from Docker on Linux, which usually stores containers and images in the /var/lib/docker directory.

The size of this file is big in your system because you have a lot of images and containers. You can reduce its site by pruning unused objects.

The easiest way to reduce Docker.raw file size is by setting the preferences, as Docker official page states:

Select the Docker icon and then select Preferences > Resources > Advanced.

Source:https://docs.docker.com/desktop/mac/space/