You would use one if you have a patch of vegetable garden, large or small that needs digging and you want to save a lot of time and effort.
You could also use a cultivator, tiller or rotator if you have a plot that needs to be prepared for sowing, a flower bed that needs weeding where you need to work between rows and weed the bed.
A Bertolini Semi-Professional Tiller in Action
Finally, you might use one if you have a patch of land that needs cultivating from scratch, breaking up and digging over ready for further cultivation*
*N.B. This would be carried out by a petrol rear tine tiller or cultivator