Spar varnish was originally developed for coating the spars of sailing ships. These formed part of the masts and rigging, so suffered a hard life in service. They were flexed by the wind loads they supported, attacked by sea and bad weather, and also suffered from UV degradation from long-term exposure to sunlight.
The most important condition for such varnishes to resist was the mechanical flexing. This required a varnish that was flexible and elastic. Without elasticity, the varnish would soon crack, allowing water to penetrate to the wood beneath. At the time, varnish production was rudimentary and had only simple materials with which to work. It pre-dated the development of modern polymer chemistry. Spar varnish was a 'short oil' varnish, where a small proportion of a finishing oil, universally boiled linseed oil, was added to a majority proportion of varnish (see Danish oil for a 'long oil' finish). This gave flexibility, even though its weather resistance was still poor and relatively frequent re-coating was required.
In modern times, 'spar varnish' has become a genericised term for any outdoor varnish. Owing to modern varnish materials, their weather resistance is likely to be good, but the original requirement for flexibility has largely been forgotten.