If you think about it it's all about minimising risk, and usually that risk is about bacteria. Milk is a really good environment for bacteria to multiply, and 21 to 47C is the "danger zone" for any foodstuff WRT bacteria multiplying.
So you want to minimise the chance for bacteria to be introduced in the first place. You wash and sterilise the bottles, you use a clean knife or the in-tub leveller thing, you don't reuse scoops from another tub, you don't take the scoop out of the tub except for a couple of seconds to deposit the milk, you don't leave the tub uncovered, you use boiled water.
Second, you want to nuke any bacteria there is in the powder which is why you use hot water to make the bottles.
Thirdly reduce the chance for bacteria to grow. Don't let bottles hang around in that 21-47 degree zone for any longer than necessary. Cool rapidly, heat only just before feeding, don't keep them at room temperature, throw refrigerated, untouched milk away after 24h, throw any milk which has been partially drunk away after 1 hour (in case of bacteria from the baby's mouth). Ideally make bottles up as and when you need them.
If you bear these things in mind you can see that common sense tells you which things are more risky than others.