When and why does our body use lactic acid fermentation?

Why? If our body's didn't have the Lactic acid fermentation it would stop some variety functions such as allowing our muscles to continue producing ATP from glucose like when oxygen levels are low, with intense exercise , or allowing bacteria to do it's thing
When? when our cells need energy , they break down simple molecules like glycolysis 
This is what is in my Biology book that is where I got my answer, Hope it helps have a nice nite 

The first statement is

Other organisms can change their behavior to take the place of the plants in the ecosystem.