Miners are receiving nearly $100K USD per day but the share to the smaller operators is often just small amounts, oftentimes $20 or so per month. The cost for them to cash out using the bank system is often expensive. This provides an economic method to cash out small amounts without incurring high fees.
More to the point, they won't actually have to cash out until they're ready to make a purchase with their proceeds. This means more money kept in Bitcoin for longer instead of being cashed out to sit in a USD/EUR/GBP bank account somewhere.
