Disney uses biometric finger scans or photos at the gate; if someone else has already used a day on a multi-day pass, it will not work for you.
Buying 3- to 5-day passes online significantly lowers the "per day" cost compared to buying single-day tickets at the gate each morning.
Trusted sites like Undercover Tourist or The Park Prodigy often provide slight discounts on multi-day tickets that you cannot get at the physical ticket booth.