>What is the use case for adding two SEDs?

I use this when testing chromatic effects on galaxies with color gradients.  Say a certain galaxy has a bulge+disk, but you can't resolve them independently.  Then the SED you might assign to the composite object from the photometry is probably roughly the sum of the SEDs of the individual components.
I could also imagine building up an SED by adding an emission spectrum on top of a simple stellar population spectrum, for instance.

>I guess I would have thought that the resulting wavelength range would be the union of the two original ranges

My thought was that outside the individual wavelength ranges, the SED is just undefined, as opposed to zero.