"The GPU reactor is meant to be an add-on board that provides additional power to the GPU for extreme overclocking such as LN2. We could have just added it on the regular PCB, but as you may notice, the PCB is already overcrowded with the existing amount of power phases thrown on. So to answer your question: this was the best way to add additional power circuitry to the card, without making the card absurdly large (i.e. keeping it within a 2-slot design). Thus, the decision was made to have it as an add-on PCB for LN2 overclocking, and for regular usage (especially SLI configurations) the user can remove it to prevent the card from taking up 3 slots. I hope this answers your question on that. For regular overclocking on air, the GPU reactor is not needed to achieve maximum OC, but it most certainly does not hurt..." - MSI USA"