Your immediate benefit is less labor. Your second benefit is your client can receive a full digital color printed banner for the same cost or less. Our cost per sqft for a printed banner is far less than what it would be for a multi color vinyl installed banner. If you are trying to justify buying a printer, a printer will open up more capabilities, you need to be able to keep that printer busy to offset the cost of running/maintaining that printer. IE INK, heads, calibration, printer payment/cost, electricity, space and if you start printing on regular vinyl the cost of a laminator and the materials associated.
If you can run a paid print job every other day atleast, you won't regret getting a printer. do your research before you buy a used printer, I learned the hard way. Get someone who has had experiance with printers to help you out with the purchase / inspection of the used printer.