Just my two bits, preparing for a test, especially when you just started aikido, gives you the opportunity to focus on the basics of the techniques. We have a set of techniques we have to perform for each kye exam. Each kye grade exam add something to the previous exam requirements and it becomes your foundation for the rest of the techniques you do during class and seminars. Through the year we do all kinds of techniques but once a month we have exam training, during the last month of the year we pretty much don't do anything but exam training. Last training of the year, there's the exam and if our teacher lets you take the exam, you already know you have passed (but you still have to get in front of the class and show it
!).
Afterwards, you get a nice certificate and everybody is happy, it is a good way to close the season and to keep people motivated to come back next season.