Ships needed tar because wood, rope, and seams are vulnerable to water. Tar helped preserve, seal, and maintain those materials in harsh marine conditions.
A traditional ship was not a single object so much as a collection of materials under stress. Wood moved. Rope stretched and frayed. Canvas wore. Seams opened. Salt water entered everything. Tar helped shipwrights and sailors manage those problems before modern synthetic coatings and fibres existed.
Tar was used on rope and rigging to slow moisture damage and wear. It could be part of treatments for timber, spars, fittings, and external gear. Related materials such as pitch were used in sealing and caulking contexts. The exact material and method varied by time, region, and vessel type.
The reason tar mattered is not that it made ships immortal. It made maintenance possible. A tarred rope still needed inspection. A tarred surface still aged. A sealed seam could still fail. But tar gave working vessels a practical layer of defence against rot, water, and abrasion.
That is why boat tar, pitch, Stockholm tar, and naval stores form one continuous family of maritime subjects. They belong to the everyday technology that kept wooden ships moving.