Crm post and pre images entity

Question

Entity entity = (Entity)context.InputParameters["Target"];
Entity preentity = (Entity)context.PreEntityImages["PreImage"];
Entity postentity = (Entity)context.PostEntityImages["PostImage"];


i Have an entity email and i am developing a plugin kindly tell me what kind of values will be fetched by entity , preentity and postentity and kindly tell me why post and pre entity images are used