192.168.0.0 through 192.168.255.255 are private IP addresses.
These are "non routable" over the Internet. If you were to just pick some random Class C, if you don't use specific reserved private IP blocks, you WILL be using some public IP of some real site somewhere...Imagine if this practice were widespread.
What if you happend to pick the IP block google.com actually uses? Yahoo? Ebay? For that reason, specific blocks in the class A, B and C range were setup to never be used over public routable networks. This makes them safe for you to use on your private LAN without worry that you are inadvertantly using someones real, routable, public IPs.