Formula feeding question

So, apologies if this is a stupid question..

I’m just unpacking our Tommee tippee bundle, bottles, steriliser, prep machine etc and I’ve got some tiny little pots called “milk powder dispensers” (???) now the instructions say that you can fill a bottle with water, put the formula in the dispenser and the dispenser will fit inside the bottle, then when you’re ready to feed you tip the formula into the bottle. It says it’s ideal for trips out etc

My question is... would you not just make the bottle up before you left the house? Doesn’t the bottle need to be warm when you feed the baby? Does it matter or is it just a different way of doing it?