Why do Feng Shui masters attach great importance to the location of Xuanwu since ancient times?

One of the four spirits, the others are Qinglong, Suzaku and White Tiger. Xuanwu generally refers to relying on it. For example, if you live on the 7th floor, the height is 10, followed by 18, or there are mountains and hills behind, all of which are basalt and can be relied on. Such as the ground, main roads, etc. It's all behind you, and it's Xuanwu Square, so you just have nothing to rely on. This is a typical layout in many geomantic omen.