Vineyard workers initially worked behind a horse and cart. The horse was trained to recognise the scratching of the rose thorns on his flanks as the end of the row. The horse would then turn into the next row allowing the farmer to continue working slowly out the end of the row.
Tractors bought about the retirement of the horse and viticulturists then started to use the rose to monitor the presence of disease in the vineyard. Sadly it was discovered that by the time the roses had the fungal disease it was almost too late to spray the vineyard. The viticulturist had to (literally!) give up smelling the roses and actually monitor the vines!!
Today roses remain as part of tradition - some vineyards will use red and white roses to indicate the different varieties of grapes but more often they are there for their aesthetics and beauty.