Leisure batteries and car batteries both make use of lead acid technology. That is to say they are both plastic boxes that contain rows of interconnected rectangular flat lead plates and a mix of sulphuric acid and water.
The main priority of a car battery is to provide starting power for the vehicle. In addition it will provide a stable flow of power for the car electrical systems. A car battery cannot be allowed to remain far below 'full charge' for any length of time without physically degrading and permanently losing performance.
The main priority of a leisure battery is the flow of power with engine starting coming secondary to that. Some leisure batteries are not actually designed for starting engines while others have both capabilities.
The internal structure of a car battery contains many, thin lead plates. The surface area* of all of these lead plates combined allows for the strong boost of power required to start the car engine.
By contrast a leisure battery contains fewer plates, each of which is physically thicker.
Each lead plate, when submerged in sulphuric acid and water is capable of storing and releasing energy. A thick plate will release its energy slowly. A thin plate will do the opposite.
Over the past century car batteries haven't changed much in their basic design. Engineers, by experimenting with different numbers of lead plates, different sizes and thicknesses, managed to create lead acid batteries for specific jobs.
So why do leisure batteries tolerate being fully discharged and why do car batteries not?
The chemical reaction between the acid and the lead plate causes atoms known as electrons to jump from the (+) plates to the (-) plates. In doing so the basis of electricity occurs.
When a car battery stands for a period of time without recharging, the thin lead plates minus their electrons will sulphate and degrade. Their performance will be reduced permanently.
Thicker plates, because of their smaller surface area* rarely lose 'too many' electrons and will not suffer the same level of destruction via sulphation. This means that a leisure battery can stand being fully dis-charged and will ultimately power your caravan appliances for a good deal longer before needing replacing.
(* surface area to mass ratio)