Diachronic change and synchronic variation are two sides of the same coin.

The question of whether variation precedes the change or vice versa is a hen-and-egg problem.

Grammaticalization is a kind of directed variation.

Analyzing synchronic phenomena as cases of grammaticalization is instrumental in understanding them (Givón 1979).

Givón, Talmy 1979, On understanding grammar. New York etc.: Academic Press (Perspectives in Neurolinguistics and Psycholinguistics).