iMac startup question

I've got a new 21.5 inch iMac (late 2013) base model. When I got the computer I updated it to Mavericks. I noticed when I turn the computer on (I usually it power it down when I'm away from home for a long time) it will show a loist of users and i must enter a password for the OS to continue loading. I was just wondering why this is happening as my old Intel imac for 2006 never did this.