If your label is going to withstand harsh conditions (rough handling during shipping, extreme hot or cold weather, abrasion, moisture, etc.) it is probably a good choice to topcoat it. Topcoating a label is a method of applying some sort of protective layer over it. There are different kinds of protective layers:
Label materials that usually need to be protected are paper and thin films.
There are other methods of topcoating a label which have an aesthetic rather than protective purpose.