Tar was essential in the Age of Sail because wooden ships and natural fibre rigging needed constant protection from water, salt, rot, and wear.
The Age of Sail ran on materials that decay. Hulls were timber. Rigging was hemp or other natural fibre. Sails were canvas. Blocks, spars, yards, boats, barrels, and fittings all had to survive wet, moving, salty conditions. Tar helped make that possible.
A sailing fleet needed maintenance supplies almost as much as it needed sailors. Tar, pitch, rosin, turpentine, rope, canvas, timber, iron, and oakum formed a practical web of naval stores. Without those supplies, ships became slower, leakier, less safe, and less useful.
Stockholm tar became famous partly because maritime demand rewarded quality and reliability. A barrel of good tar could travel from forest production areas to shipyards and harbours far away. It was a forest product with global consequences.
Calling tar essential is not exaggeration. It was one of the maintenance materials that allowed wooden ships to cross oceans repeatedly rather than exist as short-lived timber experiments.