Why is Docker best practice to minimise the number of layers in an image?
you should minimize the Layers of an Image for following Reasons:

first, you can only have (afaik) 127 Layers in an Image.
second: yes, you should include severals Steps in a Single run because of the Copy-On-Write functionality:

if you run:
RUN dd if=/dev/zero of=output.dat  bs=1M  count=10
RUN rm -rf output.dat

your image is still 10MB (plus some bytes for the delete Marker)
instead, you should run:
RUN dd if=/dev/zero of=output.dat  bs=1M  count=10 &amp;&amp;\
    rm -rf output.dat 

then, your image size is not increasing.
surely, this is not the Usual Use-Case, but, for example from a Real-World-Example:
# install terraform
ENV TF_VERSION=&quot;0.12.16&quot;
RUN curl -LO https://releases.hashicorp.com/terraform/${TF_VERSION}/terraform_${TF_VERSION}_linux_amd64.zip \
  &amp;&amp; unzip terraform_${TF_VERSION}_linux_amd64.zip \
  &amp;&amp; chmod +x ./terraform \
  &amp;&amp; mv ./terraform /usr/local/bin/terraform \
  &amp;&amp; rm terraform_${TF_VERSION}_linux_amd64.zip

more informations here: https://docs.docker.com/storage/storagedriver/#the-copy-on-write-cow-strategy