Tar can preserve some leather by adding water resistance and a protective oily coating, but it also darkens, stains, stiffens, and changes leather, so it belongs mainly to rough traditional uses rather than fine leather care.
Leather and tar have met most often in hard outdoor settings: harness, boots, working gear, military equipment, and rural repairs. The purpose was protection, not luxury finish. Tar could help shed water and add a dark, durable coating to leather that would otherwise suffer from wet use.
The behaviour depends on the leather. Thick vegetable-tanned working leather can tolerate heavy treatments better than delicate, chrome-tanned, dyed, lined, or decorative leather. Tar can darken the surface permanently, leave smell, attract dust, and make the leather feel sticky or stiff if too much is used.
Tar does not replace proper leather dressing. It is a strong, character-changing material. On some old working gear, that change was acceptable because weather resistance mattered more than appearance. On modern bags, seats, garments, or fine boots, it can easily be the wrong material.
The direct answer is that tar can protect leather in rough historical and outdoor contexts, but it preserves by altering. It should not be treated as a neutral conditioner.