The Donkey kick exercise (also known as a bent-leg kickback, as they are so like kickbacks) is an isolation exercise for your glutes. Donkey kicks are for toning and strengthening the lower body, therefore performed on your lower body split days, in a full body split programme or a way of achieving a proportionately bigger gluteus maximus (buttocks) if desired for body aesthetics.
As these are an isolation exercise it is important to use correct form else you risk your workout being less effective. No equipment needed for this exercise, however if these become too easy for you, make them more difficult by doing them with a resistance band.