Though the simplest of pp compression valve, the standard fitting does its job perfectly well. When all that is required is the joining of tow lengths of pipe, a standard compression fitting is totally suitable. There is no advantage in making the connection more complicated than it needs to be.
The biggest advantage standard couplings have over flared compressions is thanks to their simplicity and the requirement of one ring and one nut, fitting is straightforward and can be installed with the minimal of tools and fuss to achieve a satisfactory result. Most standard compression fittings can be completed with standard tools, such as a wrench to tighten the nut. They do not require soldering and can be completed by non-professionals. This also means that a standard fitting is much faster to complete than a flared fitting.
Another advantage is that when making a standard coupling, there is no need to alter pipework. When a flared fitting is required, the pipe/tube needs to be altered to match the flare of the fitting. This requires a special tool. The resulting flared joint offers a tighter, more reliable seal preferred in chemical and electrical piping and high-pressure systems such as oil and gas pipelines. As these industries have more and higher danger risks when a pipe leaks, they require the most robust fitting against dangerous and costly leaks.
On the issue of cost, standard compressions are the lowest priced option, requiring just the ring or nut and minimal labour. Another advantage is that the simplicity makes them as easy to disassemble as assemble.