Why it matters.
Control whether neutral objects appear neutral or deliberately warm or cool.
A closer look.
Auto white balance estimates a setting. Presets and custom measurements give more control. RAW processing allows broad later adjustment; JPEG has already applied a rendering.
Picture this
Photograph white paper near a window, then under a household lamp. Compare Auto and a matching preset.
Keep in mind.
One setting may not neutralise several differently coloured light sources in the same scene.