The TestDaF exam is in the B2/C1 range and therefore requires an advanced language level. It is therefore recommended that you attend a C1 course beforehand.
Not only for university applicants, but also for professionals it can be useful to take the TestDaF exam. The standardized exam is also a meaningful proof of language proficiency when looking for a job and increases your qualification.
Note that the registration period for the exam ends approximately one month before each exam.