Custom white balance

I'm sure there is a lot to be said about this, but what exactly is the point of it?

To remove color casts from your images - that means white is white, black is black and all the other colors are reproduced accurately. A custom WB used with a proper nuetral gray or white card can make sure your images are accurate with regard to color.

You set the colour of the white. It's a huge difference while shooting in jpeg and indoor under artificial light.

Best example I can come up with is go shoot a hockey game. What's white? the ice? the boards? player's uniform? Maybe all appear white, but some are likely off-white.

Tungsten, or florescent lights make a difference. Use the 18% gray card under the lighting for the 'official' balance, or set the white you want balancing off the ice, boards etc. Makes a huge difference that you can see during PP....all colours will be accurate.

The custom pre-sets can be pretty close but AWB isn't very good indoors. Notice that the very last exposure in the 2nd group of tests was of a gray card, & it's way off.
https://photography-on-the.net/forum/showthre​ad.php?t=54281

It only takes a few seconds to set a Custom WB as a starting point, but the correct WB isn't always the right WB. We used to use filters to shift the colors. Sometimes I'll shoot on the Cloudy pre-set in sunlight.
Do what you "feel" is right for the shot.

The preset WB can get close to being accurate in some situations but the truth of the matter is that no light is the exact same in different places. A great example of when it would come in handy is my kitchen area. I have a florescent light over my kitchen, halogen light under my cabinets, and an incandescent light in my breakfast area. No preset WB will work. Also is you have light coming in through a window and a lamp or overhead light in the room there is more that one color temp in the room. All in all the presets are wrong more than they are right IMO. Then you can also take into account that there are many different shades of light under a cloudy sky, a shady tree, and hundreds of k ratings of florescent lamps to give different colors ( grow lamps, cool white, ect.).

Here's a sample. The statue and the wall in the background are both "white". There was bright tungsten light from overhead and to the front of the statue and light from a north-facing window from the side/rear. Selecting tungsten gets it close but leaves a blue tint to the statue because of the light from the window. In this case the statue is white and I don't care about accurately depicting the white wall in the background so I can simply use a WB tool during post processing to correct it for the statue. If the statue had been a different color then I would have needed to select and use a custom WB setting while I was there.