Well you did just say it...eliminate all life of sufficient bio-mass...Admiral Whitcomb wrote:But 343GS said that the halo's were designed to eradicate all life within THIS galaxy.
Halo: The Flood Page 256
"This installation has a maximum effective radius of twenty- five thousand light years, but once the others folow suit, this galaxy will be quite devoid of life, or at least any life with sufficient biomas to sustain the Flood."
So maybe the forerunners wanted to eliminate all life here, to keep the flood away, then secretly repopulate this galaxy after the flood no longer had an interest in it. of course that raises the question of why the forerunners would keep the flood on the halos...
Also, they attack and consume all life, but can only be supported by calcium-based life forms...