I suppose there's a kind of hierarchy: Can I get paid to work on this software? Will customers buy this software? Is this software fit for some purpose? Is

I suppose there's a kind of hierarchy: Can I get paid to work on this software? Will customers buy this software? Is this software fit for some purpose? Is the software correct according to what the customer expects? Writing correct software is just a straightforward way of meeting customer expectations, which is a way to ensure that the software is fit for some purpose, which is a way to convince customers to buy the software, which is a way to ensure that you'll continue to be employed working on the software. It's not the only way to get paid, though. (Sometimes software is developed by end users, in which case you can skip over the "will customers buy this?" question.)