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It requires energy for the Hive Fleet to create the Biomorphs it sends down to a planet to consume. It's going to want as much of that biomass back, and thus the conquering army... assuming it is victorious... is then all rendered back down into nutrient goo to be fed up to the orbiting Hive Fleet via capillary towers. If they didn't, that's thousands to millions of tons of biomass left sitting around on a dead world with nothing to do. All those Tyranid units left on a lifeless rock with nowhere to go and nothing to do. Far better for the Hive to reclaim their biomass and recycle it to move on and invade the next planet.

These capillary towers then devour themselves from the ground up, ensuring that the Hive Fleet regains as much biomass from the devoured planet as possible.

This is why the atom-destroying weapons of the Necrons are so devastating against the Tyranid. They leave no dead to consume and recycle.

 Exergy wrote:
Perhaps you should re-examine the foolish human science you seem to know nothing about. With our primitive understanding we have been able to make biomass out of all matter in the galaxy 60 years ago. Perhaps tyranids are the primative ones, unable to create biomass out of rocks and stars like 20th century humans can.

You're suggesting we know how to create biological weapons with the full range of functions that modern Earth animals have, and that whatever method tyranids use to create the biological weapons we see in fluff and on tabletop is more primitive than what we have come up with so far.