My 558 has the same function and it is actually very handy at times.
What it does is show the reading for your flash AND the ambient (non-flash) light on the same scale, and then gives the ratio between them. You can use this information to adjust flash output to "balance" the relative amount of light from each source that will show in the image.
For example, you may want the warm effects and shadows cast by tungsten lighting in a room to show through the strobes in an image. You would then dial your strobes down to the point where the tungsten lighting effects would show through at least partially, say like 1 or 1-1/2 stops below the strobes.
Sidebar note on this meter, or at least with the 558 -- the ambient light head is so sensitive that if you use it under cheap fluorescents (cheap fluorescent lights cycle at AC line rates, or 60Hz in the US), the reading will flicker through a several stop range as the lights cycle... This fluctuation can actually trigger the meter and be "captured" if you are in the cordless flash mode(!)
