Used bikes are going to be more affordable; they’re not as much of an investment or major commitment just to get yourself on the road. Because they’ve been used by someone else, they’re also likely to have little scratches and dents on them, and when you’re learning to ride, it’s often easier to do so on something that doesn’t feel perfect and precious. This little bit of patina on the bike is likely to make you feel more comfortable using it and less like you have to baby the thing. And when you’re ready to upgrade, assuming you haven’t beat the thing up too badly in your learning process, you can sell it for close to what you paid.