Many upper level rewards credit cards have minimum monthly spending requirements, such as the AMEX Business Gold with it's $2,000 minimum. If you live a frugal lifestyle and don't have a mortgage it can be difficult to ensure you spend that much every month. You can always buy gift cards, but even easier is using fee-free Amazon Webpay to transfer money from your card to a spouse, family member, or business partner.
Amazon provides the Webpay service at no charge; since many people will spend payments received at Amazon immediately rather than withdrawing the funds to their bank, Amazon retains enough of the payments to make it worth their while. The minimum to withdraw to a bank from Amazon payments is $10, but you can send Amazon gift certificates for amounts as little as $1.
Say you've only spent $1500 for the month and your minimum spending limit is $2,000—as long as you have a spouse, family member, friend, or business partner you trust, you can send them $500 which they can then almost immediately send back to you. This can really help if you still haven't met your minimum limit after charging your groceries, gas, utilities, and other expenses to your card.