Homeostasis is the regulation of internal body conditions such as temperature, sugar levels in blood, salt content etc. Without it many functions would become non-functional. If you take temperature for example without the control of our internal body temperature many metabolic reactions (enzyme-controlled reactions) would stop/ slow down. If the temp gets too high the enzymes denature and you wouldn't get an enzyme-substrate complexes forming. If the temp is too low the enzymes have a low kinetic energy and so don't collide with the substrate as much as at normal body temp. pH has similar affect but a change in pH will always denature enzymes.