I got confused by this as well! I had the perfect prep ready for when we got home....got home and it had no power in!! We had no clue how to make a bottle up etc, as we were relying on that. Sent it back so for 4 days we made bottles, I waited 10/20 mins then added the formula! The water needs to be at least 70c so it can kill of Bactria
I presume it's something to do with the plastic bottles? Heat used to react with the old style BPA bottles and release chemicals? Most bottles are BPA free not all though.
Also, when your making a bottle with boiling water you're at risk of burning yourself? When you shake it (specially Tommee Tippee bottles) I found it sometimes it sprays at the top.
Or it may kill off nutrients in the formula!
Like I said I'm only presuming.
Perfect prep is deffo worth investing in, it's saved my sanity - if you're not planning on BF long term, id say get one! Xxx