Why is it easier to use a potentiometer in a circuit rather than two separte resistors in series with one another?

Because by potentiometer we can drop voltage according to our requirement but in set of two series resistors we can not do this.

Explanation:

As we know that the potentiometer can act as a variable resistor but resistors have constant value.

In any circuit if we insert resistors they will drop a constant value if we insert a potentiometer we will drop the value of voltage by according to our will.

Therefore, by the above discussion it can be say that the potentiometer is better than two resesters in series.

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