Namely that you
a) don’t need to be an expert mechanic, nor hire an expert mechanic to figure out its problems,
b) don’t need to worry about undiagnosable/unforeseeable problems that might come up because of the lunkhead that owned it before you,
c) have a warranty, and
d) you can break the bike in.
If you can get a new bike on the cheap, there’s nothing wrong with that. The only reason for buying a bike used is to save money, and that only holds water if there is a big price difference (which there usually is, especially with bigger, fancier bikes). However, I bought my ’09 DR-Z400sm for $5,000 (used prices were about $4,500-$5,000 at the time) and my ’08 GSX-R600 for $7,400 (two years later it’s not much cheaper: used ’08 GSX-R600 for $6,995), and I don’t regret either of those decisions.