Used bikes offer more value. The second a new bike leaves the showroom, it depreciates considerably, and every year that goes by after that, the bike will depreciate more and more. Older bikes have already seen most of their depreciation, and in some cases will increase in value.
Used bikes have more character. You don’t have to worry so much about a used bike getting scuffed up since it probably already has a few imperfections.
Buying used is good for the environment. Every time a used bike is bought, that makes one less bike going to a landfill. Steel/aluminum bikes, in particular, are great since they can be scrapped and recycled if damaged beyond repair, and steel is even better because it can last a lifetime if cared for properly! Carbon frames are not currently recyclable beyond parts salvage.