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