Let me welcome you to QuickBooks Community, @tim622.

I'll share with you some insights on why creating an invoice directly affects your inventory.

Invoices track the sale of a product for inventory control, accounting, and tax purposes, which help keep track of accounts payable. While inventory is the product that you track when buy or sell an item.

Once you create an invoice, your inventory will be reduced according to the quantity that you've used in the sales transaction. These two are directly proportional to each other. Every time you use your inventory item on sales, the amount on its quantity will decrease. 

To successfully track inventory and know more about how QuickBooks handles inventory assets, please check out this article: Understand inventory assets and cost of goods sold tracking.

I'll be around if you need further help in managing your company file in QuickBooks. Just place your comment down below and I'll get back to you. Have a great weekend and keep safe.