It depends on what sort of business you're running. If you're running a graphic design, print design, or architecture business, owning your own printer, preferably 11"X17", is highly beneficial. Basically, any kind of design work really needs a printer. It's useful for sewing, of all things.
Mostly, however, having your own printer saves time more than anything else. It's vital for college students who have to print out their work fast and get it to their professor on time, and it's a convenience to not have to go to the print shop every single time you need to print something off. This can save you money in gas and labor cost, but then you have to balance that with the cost of ink and paper for the printer. That equation will sort out differently for each person's needs.