Why must all conditions except the independent variable be held constant for all experimental treatments?

Uhm... I believe you mean a situation when we examine a relation between two variables. In this case all other conditions must remain as constant as possible because they could affect a result and then the conclusions could be false. ie. you examine a relation between temperature and frequency of cell division/organism's activity, then you should stabilize air pressure, food composition, bacterial flora in experimental room... etc. of course when you conduct research in a field, ie. ecological research you can't do any of that but then you are obliged to explain possible aberrations of your results in an article's discussion.

Explanation:

The relationship of forms and functions forms the concept of cells physiology.The cells of the body were created to get adapted to specif functions.

Neuron is an example of cells  with forms and function related.Neuron are the structural and functional unit of nervous system.They are made up of the  cell body which contain  nucleus, the long cylinder called axon,and the end terminal or end button. The cell body  control the whole neuron,and receive impulse  from receptors or other neurons through the dendrites

These impulses are transmitted to reach the target or effector by the nerve or axon of axonplasm  surrounded by myelin sheath.Therefore desired information are transmitted from the CNS to reach the target part of the body to bring about response.

On reaching the targeted part of the body,the impulse cross gaps of synapse through the end buttons to reach the effectors(glands and muscles)

The myelin sheath are interrupted at intervals by gaps called Node of Ranvier. These makes the impulses to jump and therefore increases the speed  of nervous transmission as salutatory conduction.This adaptive feature ensure fast transmission of impulses and therefore aid the functions of neuron.

The cones and rod cells (photo receptors)of the eyes are another example. The rod are rod like in shape and are for twilight vision or day vision. Because they are needed in large number for the day vision they are at the boundary of the retina for maximums light reception,The cone shaped cone cells  are fewer ,and for colored visions,They are located deep in the retina.