Focus stacking/bracketing consists of taking several shots of the same frame (in this case, a landscape) at different focus points within your frame to ensure sharpness throughout. Is it necessary? In most cases, no, as an aperture above f/9 will typically cover any slight variations in depth of field. However, if you are using an ultra-wide lens with subjects close to the lens and a large distance between the foreground and background elements, it is beneficial to ensure sharpness throughout.