A power function is basically a variable base raised to a number power. For example, a power function is basically a function where where n is any real constant number.
Polynomial functions are basically a sum of power functions - nothing more than that.
Polynomials have certain properties which are really power-like.
When a variety of different power functions get added together, polynomials tend to take on certain unique behaviors.
A polynomial may comprise of a large number of power functions, but on of them will dominates all others eventually.
In a nutshell, the end behavior of both power function represented by the leading term - the term containing the highest power of the variable is called the leading term of the function - is very much is the same as the end behavior of a polynomial function.
The line is often referred to as the "line of best fit", this line is drawn so that more accurate conclusions can be drawn from given data. This has numerous uses across the sciences.