How does having a full disk "slow down" your computer?

When you search on Google for ways to optimize computer speed, in almost all lists you see something like this -

3) Remove unnecessary files from your disk to make your computer run faster.

This 'advice' is literally everywhere. Everyone from online article and my father believe that full disk = slow speed.

I don't understand how -- the disk is only used for storing long-term and temporary files, the RAM and CPU are what determine computing speed! I thought about paging, but that does not seem to be a viable explanation.

So if anyone could clarify how full disk = slow computer makes sense, I would be thankful!

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