Tar helps protect wood from rot by slowing water movement into the surface, reducing repeated wetting, and leaving a durable coating that discourages decay conditions on exposed timber.
Wood rots when fungi have the right combination of moisture, oxygen, warmth, and food. Tar does not make wood immortal, but it changes the surface environment. A tarred surface sheds water better than bare wood, dries more slowly in some conditions, and is less inviting to the organisms that thrive on constantly damp untreated timber.
Traditional wood tar also carries smoky and resinous compounds from the material it was made from. These can add some preservative value, especially when the tar is used on outdoor timber that would otherwise soak up rain. The effect is strongest when the tar reaches the outer fibres instead of sitting as a loose skin on dirt, old paint, or wet wood.
Tar protection depends on the species of wood, the quality of the tar, how deeply it penetrates, and how much weather the timber faces. Softwood posts, fences, sheds, boats, and exposed fittings were common uses because they needed regular, repairable protection rather than a perfect modern seal.
The important limit is that tar cannot save wood that is already structurally rotten or trapped in constant damp. It works best as part of maintenance: applied to suitable timber, allowed to settle, inspected over time, and renewed before bare wood is exposed again.