Answer
When we think about the rate of change of a function on an interval, say $[a,x]$ we ask how much its value changes per unit length of that interval i.e. we divide the change of the function $f(x)-f(a)$ with the length of that interval which is $x-a$ which is exactly the slope of the secant.
Work Step by Step
The slope of the secant line through $(x,f(x))$ and $(a,f(a))$ is given by
$$m_{sec} = \frac{f(x)-f(a)}{x-a}.$$
When we think about the rate of change of a function on an interval, say $[a,x]$ we ask how much its value changes per unit lenthg of that interval i.e. we divide the change of the function $f(x)-f(a)$ with the length of that interval which is $x-a$ which is exactly the slope of the secant.
