Why is this restriction so useful in economic analysis? Because one of the two variables might have a larger impact on a third variable than the other one being examined.

If other variables that do not interact with these two variables are not held fixed, the true relationship between the two
variables would be hidden.

To fully understand the relationship between two variables, it is necessary to change each of the variables sequentially
while holding the others constant.

If every variable is allowed to change, it would be impossible to isolate the impact of one variable on another.