Why monetise something worthless, like CO2,

The exchange platform for credits and offsets is a fraud legitmated by government monopoly. Participants assume no liability by issue or trade of such securities --certificates that represent interest in a rated reduction or promise to reduce CO₂ emissions purportedly by means of plant, equipment, foregone energy intensive activities, ownership interest in third-party foregone energy consumption, or all of the above. And apart from inadequate regulatory metrics and practice, we find the "face value" (cash value of maximum emission volume) assigned such instruments is not legally binding in each of the jurisdictions where exchanges are established. In short contracts implying buyer's promise to extinguish CO₂ production are not executable.

More to the point of your citation, the frequency with which participants plow back rather than hold or distribute proceeds obtained by sale of credits and offsets is predictably low, because the security per se, not the underlying "asset" CO₂, is a store of value and fungible. More plausible (analogous to buy-back behavior exhibited over the past decade), transnational holding companies may use proceeds to purchase properties (e.g. subsidiary entities and real estate) to hedge the price of credits held and credits available for sale.

The purpose of the exchange is to introduce an additional source of liquidity to existing primary and secondary money markets.

when you can monetise energy value quite straightforwardly?

You know the answer is simple enough: Producers do not voluntarily reliquish profit, and no government requires they must.

Diversity is the key to economic and political evolution.