There is no right tempo for use all the time. For example, slow tempos can be very beneficial for beginners to build up the mind-muscle connection.
A more advanced person might use a slower tempo as part of a pre-exhaust for the targeted muscle(s) of the training session. This can be really useful for a lagging body part or even working around an injury. For example, if you are coming off a knee injury but really want to hit your quads, you might do some light pre-exhaust work on the leg extension before squatting or the leg press.. then you can hit that muscle group "hard" without using nearly as much load.