What is the purpose of Dockerfile and Docker-compose in development mode?

I am building a MERN stack app. but I am not sure about the benefit of dockerize it on development mode.
I saw many examples on github where there are separate Dockerfile on client and server folder and there is a docker-compose file where they import client and server services. but do I really need them?

If I need then, how can I run command parallelly? suppose I need to build the backend in watch mode in one terminal and in another terminal I need to execute the yarn start command. How can I achieve that?