What s the point of preheating an oven?

I was waiting for an invoice to be sent to me yesterday, and I didn't get it. So I just wondering if you could send it to me? I just need an itemized list of the parts his buying. So that way, I know and have it on my record. What I'm looking for is the invoice; and I know you got to order some parts, and I think he said it might take him until Monday. So I just need the itemized list of all the parts of what he's buying and their price. Also, I need a coordinate somebody to be there for him.

How to replace the door switch of a Whirlpool LER3624PQ1 electric dryer?

How to reset a Frigidaire Gallery series washer and dryer?

What speed should I use for KitchenAid ice cream maker?

How to replace a KitchenAid dishwasher motor WPW10757217?

Do you guys check refrigerators to make sure they are working okay? There's a chemical smell coming into my refrigerator, but it's not coming from the freezer. My Hotpoint refrigerator is really old, and I rent, so I'm trying to establish whether or not we need a new refrigerator. Do you know anything about that? Like if it starts to smell? How much would that cost?

What should be included in my maintenance checklist for the Lennox XP25 Heat Pump?

Is it okay to adjust the temperature settings on my Takagi TK-340X3-PIH water heater as needed?

Is it okay to use the Lennox 16HPX Heat Pump as a primary heating source during winter?

How to fix a microwave turntable that is not rotating in my Wolf microwave oven?