Explain why a technique works

You follow the step every time and have never known what it is for.

Explain the technique below so I understand it well enough to know when to break the rule.

1. **What is physically happening.** The actual mechanism, in plain language: what happens to the proteins, the water, the starch, the fat, the sugar, the heat.
2. **What it achieves** in the finished dish, in terms I would notice when eating.
3. **What happens if I skip it**, specifically. Not "it will not be as good" but what exactly changes in texture, flavour, or appearance.
4. **When it does not matter.** The cases where this step is cargo cult cooking and can be dropped, if there are any. Say so if there are none.
5. **The most common way people do it wrong**, and how to tell from looking.

Where the popular explanation for this technique is known to be wrong, say so and give the better one.

Technique:
Context I use it in:

Technique and recipes. Why a step works, what a recipe is not telling you, and writing down the dish you cook from memory.