Once you’ve decided you want a glossy label, you have two options: glossy varnish or glossy over-lamination. Of course, to make a label extra durable, you can choose both: gloss laminate over glossy label stock.
Labels with gloss varnish probably outnumber all others. The varnish has a protective function that keeps labels safe from scuffing.
Glossy laminate is not as frequent choice for labels as varnish, but it is extremely useful when labeling products that are exposed to severe conditions: outdoors storage, corrosive materials, rough handling, tearing, scuffing, etc.
Also, some people choose glossy over-lamination for an extra glossy appearance of their label.