The placement test will help you define your level before starting studying with busuu.
You may want to learn French for example, but believe you already know the basics and do not want to start from the very beginning. The placement test takes about 5 minutes and will tell you where you should start in the course.
Alternatively, you can decide to ignore the result of the Placement test (or even not take it at all) and start from the beginning.
If you have started learning on busuu and wish to take the Placement Test at a later time, you can do it by accessing your Settings>Account>Take a placement test.