Question

When graphing a linear function in slope-intercept form, why do you have to plot the y-intercept first? Why can't you use the slope first?

Answer 1

Because the slope doesn't tell you any actual points on the line. 
It only tells you whether the line tilts up or down, and how steeply. 
But the line could be way up there or way down under, with the same slope.
You can't actually DRAW it until you know some point on it, and
   the y-intercept gives you one.