Tar repels water because it is oily, resinous, and difficult for water to mix with. When spread on a surface, it forms a water-shedding layer that helps rain bead, run off, or soak in more slowly.
Water and tar do not combine easily. Most tar mixtures are rich in hydrophobic compounds, which means they resist mixing with water. That is why a fresh tarred surface often looks glossy and rain can gather on it rather than disappearing into the material beneath.
On timber, this matters because repeated wetting and drying cause swelling, cracking, surface erosion, and decay risk. Tar reduces the speed at which rain reaches the wood fibres. It also fills small surface checks and roughness, so water has fewer direct routes into the surface.
Tar is better described as water-repellent than perfectly waterproof. A thin or weathered coat can let moisture through. Gaps, cracks, end grain, joints, and fastener holes can still admit water. If water becomes trapped behind a coating, it can create problems rather than prevent them.
The practical behaviour is simple: tar is useful where a surface needs to shed water and stay maintainable. It is less reliable where the job demands a continuous engineered waterproof membrane with no gaps or maintenance tolerance.