Is there some purpose to having income accounts in purchase invoice view

While I was doing reports today I discovered that on one of my purchase invoices, I had somehow selected an income account instead of an expense account for the transactions.

This raises two questions.

Is there some reason for having the ability to select income accounts in a purchase invoice and for having expense accounts selectable in a sales invoice?

If not, could the drop down list for accounts be filtered to show only income or expense accounts depending on whether your are creating a sales or purchase form?

Sometime you might purchase something that should offset your income rather than increase your expenses.

For example, delivery costs. Let’s say you have Delivery fees account which is an income account because this is the account you are using to charge clients for delivery.

When you receive an invoice from courier company, you might want to offset this cost against your income account.

Thats an interesting idea for delivery fees. I might look at that for my delivery fees problem. The problem with having the ability to put an income account in say a purchase invoice is when you accidentally add the transaction to an income account instead of an expense account. I did not realise until today what I had done wrong and I only noticed because I was doing some reports! Anyway, I was just wondering why it was like that. Thanks