What is the point of WORKDIR on Dockerfile

Question

Docker is something I'm learning. I've seen the WORKDIR command in Dockerfile numerous times:

FROM node:latest
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app
COPY package.json /usr/src/app/
RUN npm install
COPY . /usr/src/app
EXPOSE 3000
CMD [ “npm”, “start” ] 

Isn't it possible to just leave out WORKDIR and Copy and have my Dockerfile at the root of my project? What are the drawbacks of this strategy?