CO2 plays an important role in metabolic reactions, allowing committed steps to be produced. The loss of a carbon from a metabolic pathway is a high energy process to reverse, and so prevents the reversal of pathways at that stage. This allows a control point to be present, ensuring a reaction only goes in the intended direction.
CO2 is easily lost from reaction sites, as it is gaseous and able to diffuse into the blood or air. Due to the negative $$\Delta\text{G}$$, reactions liberating CO2 are energetically favourable.