However, my photos nowadays are all raw. So, when I open them, Photoshop forces me first into Adobe Camera Raw.

That's because the information in a raw image file must be interpreted before it can be converted to an image that is anything like what you expect to see on your screen.

Why Use Adobe Camera Raw?

There is no such thing as "The" raw image, there are only possible images that can be produced using the information in the raw file, which is a set of single linear, monochromatic luminance values for each of the photosites (a/k/a sensels, pixel wells, etc.) on the sensor.

Demosaicing to account for the differences due to color filters in the Bayer mask (even when producing a B&W image - remember how color filters altered the brightness of different things by different amounts when we used them on our cameras with B&W film?), gamma correction to convert the linear values to the logarithmic ones we need so that the image does not look more or less like solid black, color temperature, white balance correction, etc. must all be applied to the raw sensor data before it begins to look anything like the thing we took a picture of.

That is what ACR, or any other raw conversion application must do: convert the filtered monochrome luminance values for each sensel into a viewable image by applying color demosaicing, gamma correction, color temperature and white balance correction for various types of light sources, etc. It has to use some value for each of those setting to do so.

If you center everything to zero, you're basically telling Adobe Camera Raw that the photo was shot and properly exposed under more or less ideal conditions: Sunlight with a color temperature of around 5200K to 5500K, no harsh shadows, etc. ACR will apply default gamma curves, color channel multipliers, contrast adjustments, etc. optimized for those assumed conditions. If your light source(s) was(were) significantly different than what the default assumes, then those setting will not be correct to accurately reproduce the color of the scene.