What is pip's `--no-cache-dir` good for?

Question

I've recently seen the --no-cache-dir being used in a Docker file. I've never seen that flag before and the help is not explaining it:

 --no-cache-dir              Disable the cache.

1. Question: What is cached?
2. Question: What is the cache used for?
3. Question: Why would I want to disable it?

Answer 1

1. Cached is: store away in hiding or for future use
2. Used for

• store the installation files(.whl, etc) of the modules that you install through pip
• store the source files (.tar.gz, etc) to avoid re-download when not expired

3. Possible Reason you might want to disable cache:

• you don't have space on your hard drive
• previously run pip install with unexpected settings
  • eg:
    • previously run export PYCURL_SSL_LIBRARY=nss and pip install pycurl
    • want new run export PYCURL_SSL_LIBRARY=openssl and pip install pycurl --compile --no-cache-dir
• you want to keep a Docker image as small as possible

Links to documentation

https://pip.pypa.io/en/stable/topics/caching