Delidding is often done by people who are overclocking their systems to the max. It's done to get closer to the chip with a heatsink. Here's more: [http://deliddedtech.com/2016/04/10/the-rockit-88-delidding-tool-will-make-intel-cpu-delidding-easy/|The Rockit 88 Delidding Tool Will Make Intel CPU Delidding Easy]
It is not something I would recommend for a normal running system! You do risk damaging the chip.