Both the altitude and the perpendicular bisector intersect the base of the triangle at a right angle (90 degrees). There are 2 distinct differences between the two:

1)For the equation of the altitude the line is drawn from the vertex (at the top of the triangle), whereas for the perp. bisector it is not drawn from the vertex.

2) The perpendicular bisector intersects the base of the triangle at the midpoint (which is why you use the per. slope of the median). The equation of the altitude does not cut the triangle at the midpoint.