there are basically 2 types of ip addresses- static and dynamic. as the name suggests, static ips remains unchanged, and are generally given to people with lots of cash in their pockets and major organizations etc. dynamic addresses are given to common users like us.
eg for static ip: website servers like thinkdigit servers
eg for dynamic ip: the ip given to you in a dialup connection, and most of the cheap broadband services in india. (like airtel, bsnl dataone etc)
and if anybody uses a leased line for internet from the government, they can get static ips.
the dynamic ip doesnt change everyday. instead, you get alloted a random address everytime you conenct to your isp.
it sure is nice your ip changes. suppose you take some forum, lets take gamefaqs forum, and you post offensive messages, you will be banned for life, and your ip address will be traced so that you cannot ever login to your account, and not even you can create another id. so, when you disconnect and reconnect, then your ip changes, and so you can create an account in that forum and flame other members to your heart's content. this is not possible in static ip.
there are more advantages, but my hands hurt from that extensive typing, so im taking my sweaty hands off the keyboard.