Painting is nothing more than transfering paint to a surface and spreading it to an even coating and filling small gaps and crevices. Paint can be transferred using a brush, roller, pad, or sprayer. Each of these tools has its role. The brush loads or transfers the least amount of paint but does the best job of filling crevices and difficult surfaces like the grooves in T-111, board and batten, underside of laps and V-groves in shiplap. The roller and paint pads trasfer or load more paint per dip, and the sprayer directly transfers paint rapidly.
Backbrushing is used with sprayers and rollers to work the paint into small crevices like knots, small cracks, and to spread the loaded paint into deeper grooves and surface details.
I spray with a SprayTech EP2105 3000 psi airless. This transfers lots of paint very quickly and evenly, but the particles do not fill completely unless the first coat is backbrushed. After a smooth surface is obtained with the primer, backbrushing is used with spray to fill detail (grooves, and surfaces perpendicular with the spray pattern). The sprayer loads paint onto surfaces parallel with the spray pattern. Side and detail can be coated by angling the spray pattern, but backbrushing completes the job. Backbrushing is fast because the paint has already been transferred onto the surface to be painted, so no dipping.
Same principle with a roller. The roller transfers paint, the brush spreads it where the roller does not cover. Pads I use mainly for gutters, trim and other smooth flat surfaces. It transfers and spreads a smooth layer and is easily controlled.
Hope this helps.