I recently heard a good analogy for why we need CDR (though I forget from whom I heard it...): In this analogy, a ship (analogous to the Earth) hits an iceberg (analogous to climate change), and the ship develops a crack in its hull and begins taking on water. The ship has two options to remedy this situation: 1) patch the crack in its hull (analogous to GHG emissions mitigation), or pump out the water the ship has taken on (analogous to CDR).
When deciding which option to pursue, it is important to consider not only the costs of fixing the cracked hull vs. pumping out the water, but also how long each approach will take to implement and much water the ship will take on before the solutions are fully implemented. Even though patching the crack in the hull might be considerably cheaper than pumping the water out, if we take too long to install the patch, the ship will sink unless we also have a strong pump to bail out the water the ship has already taken on.
This analogy makes clear the option value inherent in CDR. In an ideal world, we don't have to use pump to bail out the ship because we can patch the hull quickly. But if we take too long to patch the hull, we would be wise to have a pump ready to turn on.
Today, global emissions are growing (meaning the crack in the hull is growing larger), and we have barely begun to develop cost-effective CDR solutions. It seems pretty clear to me that it's high time to start investing in both GHG emission mitigation techniques and CDR.