Why are discharge tubes filled at reduced pressure?
Not only does it take a high starting voltage and a current-limiting device (e.g. inductor) to maintain a continuous discharge at higher pressures, but the quality of the light emitted is changed by atomic (and ionic) interactions, tending to broaden lines into a quasi-continuous spectrum.

A good example is the sodium-vapor lamp, often used for street lighting. In the example cited, compare the line spectrum of a low pressure lamp with that of a high pressure one (though there is also a contribution from other gases in the second image) .

The low-pressure hydrogen glow discharge corresponds well with calculated values and is a useful tool in showing how theoretical calculation of orbitals can be translated into an observable spectrum. Calculation of high-pressure phenomena is much more difficult.