Measurement of static pressure

What is the logic behind using a wall tap/piezometer to measure the static pressure. Is it that near the walls, the fluid velocity is zero, because of the no slip condition and hence the pressure measurement near the wall would only be static pressure. There is this point in the book that i am consulting, it says "there is no pressure variation normal to straight streamlines, this fact makes it possible to measure static pressure in a flowing fluid using a wall pressure tap". Does this point mean that because there is non-variance of pressure in normal direction to the straight streamlines, thus giving a single value for static pressure, whereas in regions of curved streamlines, we would have progressively increasing values as we go away from the center of curvature of the family of curved streamlines. Also, why do we require a separate static pressure measuring instrument alongside a pitot tube (for fluids passing through a pipe) in order to measure velocity through the Bernoulli eq. Why can't we calculate velocity directly as is done, for free streams of fluids, such as river water.