Water swells the wood fibers and they will plastically (permanently) deform. This leaves a rougher surface than what was prepared by sanding or scraping. Sanding after staining typically causes the color to be more uneven, and needing another coat of stain to even out more. Pre raising the grain with plain water and lightly sanding with ~320 grit to remove the raised grain (after drying) will allow a WB stain to be applied and have a smooth surface after drying to top coat. If no stain or dye is used, then there is really no advantage to pre grain raising for a WB topcoat. The 1st topcoat can be lightly sanded instead.
It is best to flood the surface until no more water is absorbed (a spray bottle and foam brush work well), and then wipe off and allow to dry. This ensures that when the WB stain/dye is applied the level of absorption by the wood fibers does not exceed the previous wetting, ensuring no further grain raising occurs. I will typically raise the grain on softwoods twice, flooding the surface the 1st time, sanding, then wetting more lightly the 2nd time. Softwoods soak up enough water to raise large areas of grain, and the swelled fibers in those areas are removed by sanding flat. The 2nd application addresses these areas.