I guess that at one point,
intended to generate beam textures using these parameters, but it either didn't work or looked ugly, so they used textures instead.
It could actually be a nice feature, having FSO generate a gradient from +RGBA Inner: to +RGBA Outer: instead of using a texture for a beam section. Perhaps some mods could use it for beams that don't need elaborate effects due to a design choice. It'd be easier to tweak than a bitmap, that's for sure.
BTW, if they were always optional, then what does this nightly change in them?