To start of the list of reasons you should buy a arduino #1 there just flat out awesome. The type of arduino that i use the most is the arduino uno.
#2 you can do anything with arduino from blinking a led to making a animatronic Friend.
#3 you will learn how to program.
#4you will use it almost every day.
#5you will be able to do hundreds of arduino projects .
#6 you can buy my led cube shield.
#7it will give you endless hours of entertainment and fun.
#8you could make your own shield and make money off of it.
#9it will open a whole new world to you.
#10you could make it respond to twitter or tweet.
I could go on forever.