Obviously, the primary motivation for a scanner printer combination is that it saves space. If you're like most people, your home office is probably already crowded with technology. You want a printer that is as sleek and efficient as possible, not something that only works occasionally and takes up half your desk.
At one point, scanner printer combinations were significantly more money than a simple inkjet printer. With improvements in technology however, the price on these machines has come down substantially. The cost difference is now fairly minimal. In fact, the different between regular ink jet printers and printer scanner combinations is often $50 or less.