The motivation is following: Fluid movement is based on the fluid column difference (and inertia). Fluid flows from fluid box with higher level to those with lower levels. Easy right
However there are some special cases that need to be dealt with. For instance the chemical plant / oil refinery inputs should "consume all the liquid" that is coming in. That is why the concept of the base level was introduced. You can imagine that base_level -1 means that the fluid box is located 1m underground. This way anything coming from the pipe (no matter how low the fluid column is) will go in (and not back). The same works for base_level 1 or fluid box located 1m above the ground. It will always push the water to the connected pipe no matter how high the water level there is.