The reboot's necessary because with very few exceptions (like, I think, the GUI), most of avast is resident as services and EXEs, and the reboot is the only way to get the updated versions into active memory. On most systems, anyway.
Generally I'll select not now simply to give me a chance to do a "clean" logout-disconnect from my ISP and maybe also a quick last-minute check for email before doing that, and restart right after that.