What is the point of WORKDIR on Dockerfile?

I’m learning Docker. For many times I’ve seen that Dockerfile has WORKDIR command:

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” ] 

Can’t I just omit WORKDIR and Copy and just have my Dockerfile at the root of my project? What are the downsides of using this approach?