It is for calculating your cm/360 (or inches if you set it to inches) and keeping it consistent across multiple games. That and if you want to set up your sensitivity like someone else.
For example I use 1600dpi mouse, 1.14 sens in cs:go. Now, I want to play fallout 4, but fallout 4 game engine use a different formula for mouse sensitivity. Instead of messing with my aim from switching between the two game, I can use their convert calculator to keep it consistent. 1600dpi w/ 1.14 in csgo = 0.0066 in fallout4.