This is necessary for a practical classification that can be used by others. All the individuals in a given species are not identical. Think of it as a population in which any character might be expressed to different degrees in each individual, sort of a bell-shaped curve. Humans are classified as the single species Homo sapiens, but we certainly are not morphologically identical. Check this idea out by taking a stroll in the woods and examine plants of a single species.
Species is abbreviated sp.
Variety, abbreviated var. is subordinate to species. For example, the common honeylocust in the wild has thorns, but thornless plants are found. The honeylocust is Gleditsia triacanthos whereas the thornless honeylocust is named Gleditsia triacantho s var. Sometimes subspecies is used in place of variety. The term form is apparently little used by present-day taxonomists, but it is still used in the horticultural literature.