You use the change plates in combination with bumper plates, which can increase the weight on the barbell in small steps each time. Because the change plates have the same diameter opening as bumper plates (50.40mm), you can easily place them on the outside of the bumper plates.

Technique plates are designed to train technique of Olympic weightlifting movements without having to load the bar with heavy weight.

Do you need change plates? Probably. If you are new enough to lifting, you can usually get away without having change plates for a while since bigger jumps in weight are more common as a total beginner, but ultimately you'll need them.