There is a reason!

[Neoriceisgood]

If you can't make the sandstone look like sandstone with a hue shift, you're doing something wrong.

Hue/saturation shifts are important for a few reasons:

1. light has a colour, generally the lightsource produces a more yellow hue on the lightest shades, invariably making the shadows seem some shade of blue in contrast. (this is under the assumption that the lightsource is yellow or a related colour, of course.)

2. light and saturation are a fine mix indeed, shadows generally feel a bit greyer and duller than the highlights, they're not just the same colour but darker.

3. Honestly? It just looks better.

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Now on your argument that your colours represent sandstone well, that's a complete lie, google search for sandstone statues; I don't see anything that even comes close to the colours you picked.

Trust me on this, people are telling you to apply hue shifts and some minor palette changes for a reason, once you'll get a hang of it you'll never stop using it when it comes to doing art, it's that fundamental to colouring.