It comes with a serial number because the makers of any sort of computer software derive great joy out of making frazzled mothers everywhere keep track of 999999 serial numbers in addition to passwords that MUST include things that aren't in our normal passwords - 

and now some programs even have this doohickey called a dongle - I looked into lightwave until I realized I'd have one more thing to keep track of......

all makers of software are ex-husbands and they are evil - bottom line.  Models included.

I am:  aka Velocity3d