When I’ve materials for a job and I’m creating a receipt for expenses. What should the reason for purchase be cause all the reasons available are ridiculously

You don't need a specific "reason for purchase" field when recording job materials. QuickBooks relies on Expense Categories and Customer/Project tracking to correctly log job costs. 

If the dropdown options don't fit your situation, you can use the Memo or Description field to type out your reason for the purchase. Make sure to use the Customer/Project field on the expense form and check the Billable checkbox if you want to pass it through to an invoice.

For further guidance, check out this article: Record an expense.

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