it is a biological system that rather than finding its own solution to a problem, utilizes one that is a at least visually similar to the more technological approach.
They are the most effective kind of bioship, and probably the hardest to create. Although grown they are not necessarily still alive, wither in part or whole. Because of this they can have greater performance. Structures can be 'layered' in a kind of biological 3D printing. Coral-like material could be used in rockets, reinforced by fibres on the outside, and cooled by transpiration. It also makes them more resistant to temperature, radiation, and damage. They don't need feeding, medical care, or a controlled environment. And I imagine it is far more comfortable for the crew than the inside of a living organism. Of course it loses the ability to heal, but as this is going to be slow in any case, the loss is probably worth the improvement in performance. It might also be possible to 'reactivate' parts of the ship when they are damaged. Of greater concern is the fact that many biological materials loose strength when dead. Many devices such as rotary pumps can be used, which would be hard in a living system, and weapons in particular should be easier. Sensors and drives should also benefit by the greater degree of optimisation offered by not having living material.