Tar preserves natural rope by coating the fibres, reducing water absorption, slowing rot, adding abrasion resistance, and helping the rope survive outdoor or maritime use for longer.
Natural fibre rope is vulnerable to water, salt, dirt, mould, and wear. Hemp, flax, manila, and similar materials can weaken when they stay damp. Tar gives the fibres a protective coating that slows wetting and makes the rope less exposed to the conditions that encourage decay.
Tarred rope is also physically different from plain rope. It can feel stiffer, darker, heavier, and more weather-resistant. The tar helps bind loose fibres and reduces surface fuzz. In rigging and outdoor work, that could be worth the added smell, staining, and handling mess.
The protective effect is not unlimited. A rope that bends constantly, rubs against fittings, or is soaked repeatedly will still wear. Tar can also make a rope less pleasant for clean handling and unsuitable for some modern uses where grip, inspection, or compatibility with equipment matters.
Historically, tarred rope made sense because natural fibre rope was expensive, essential, and exposed. Tar gave it a longer working life in hard conditions, especially where regular maintenance was expected.