Eli5: What is the purpose of downshifting for a semi-truck and how does it work?

Question

I have always wondered what those loud noises are the semi-trucks make as they appear to be slowing down, but I don’t understand how that helps?

In: 547

Answer 1

Using the engine as part of breaking is essential for small engines like ATVs and Motorcycles where they have sequential shifters. However, it is best to keep the engine RPM lower to reduce engine and transmission usage.

I would say it is not bad, rather it is not necessary for most vehicles.