Which speed setting of a mixer-grinder should be used for which purpose? Slow, fast, medium are the speed settings in those appliances, for example this mixer-grinder. 

Why are there 3 settings, why not only fast? Which setting should be used in which case?
This is for mixer. 

At the low speed you have stir / mix.  At the high speed you have whip.

You would stir eggs to make scrambled eggs.  And you would whip cream to make whipped cream.