The best part of the extension cord was it removed that micro vibration you feel while it is charging which now exists due to inefficient grounding from the power adapter without the extension.
Yes, noticed this with my M1 MBA. I had purchased the extension cord when I bought the MBA, but wasn't planning on using it regularly. Those vibrations were so uncomfortable and distracting that they made the extension cable a necessary accessory for me.
What I find ironic is that people are willing to spend thousands and thousands on a computer but complain about a $19 USD extension cord.
Also, Apple tells you exactly what's in the box: