Vikings used tar because wooden ships, rope, and outdoor timber needed protection. Tar helped preserve and water-shed the materials that made long-distance travel possible.
The Viking world depended heavily on wood. Ships, buildings, tools, chests, fences, sledges, carts, and household objects all needed protection from weather and wear. Tar was one of the most useful materials available for that purpose, especially in northern forest regions where the raw material could be made locally.
Viking ships are the clearest reason tar mattered. A longship or trading vessel was a living maintenance problem: planks, seams, ropes, oars, and fittings all had to survive water, salt, sun, cold, and storage. A tarred surface was not maintenance-free, but it gave timber and fibre a better chance.
Archaeological research has shown that tar production in Scandinavia could be organised on a large scale. That matters because it suggests tar was not just an occasional farm product. It could be part of the material infrastructure behind seafaring, trade, warfare, and settlement.
The phrase Viking tar usually means tar in a Viking Age context rather than a chemically unique substance. It points to the practical world of forest work, boats, smoke, pits, kilns, and repeated maintenance.