This is a very simple question to answer actually. Reactor power is all about controlling the number of neutrons. Control rods absorb neutrons to help control and lower power. But if you pull out a rod to raise power, it doesn't work well if water fills in that space because water is great at absorbing neutrons too. So the designers had a very bad idea of adding 4.5 meters (over 14 feet) of graphite to the bottom of the control rod because graphite adds positive reactivity compared to water. When the rod is pulled out, graphite takes its place. I find saying the "tip" is graphite causes confusion because it's actually a long rod of graphite nearly the height of the core entire.