Answer
Because it provides more reliable results. The least-squares regression line will pass through a larger range of values of the explanatory variables (and the standard deviation of $\beta_1$ will be smaller).
Work Step by Step
Example: in the example given in this section (Total Cholesterol vs. Age) suppose that the reseachers had selected 14 females from 25 to 30 years old. A regression line found with these data would not be reliable to predict the cholesterol level of a 60 years old female.
