Cameras have Custom White Balance why could we using White Balance Filter?

Answer 1

With film cameras, filters were the only way to go. With digital however, the WB dial will accomplish the exact same thing - remove unwanted color casts.
I often use this feature when I'm shooting in artificial / low light. Some cameras only have a few presets, and these might not cover all situations, but advanced cameras let you set the WB in Kelvin or take a reading of a white / gray surface for a custom WB.
I personally only own 2 filters for my standard zoom: a UV filter to protect the front element and a circular polarizer. The only other filter I could imagine needing is an ND grad. All other filters - color filters, special effect filters, etc - can be reproduced by camera settings or in Photoshop.

Answer 2

Mainly because filters don't actually correct the colour, they filter out other colours.
For example, if you are under a standard incandecent light bulb, colours look on the reddish side because there is no blue in the light source. Adding a blue filter doesn't actually correct this. The blue filter blocks out the red end of the spectrum. So now you will have a light source that has no blue AND no red.
All you have left is cyan & green in the picture.
It's much better to have as much of the spectrum as possible hitting the CCD chip and using the camera's internal white balance system to increase or decrease the clours as needed.

Answer 3

I don't understand your question.

If you want to know why someone would change the White Balance Manualy it is because the camera can be fooled by certain lighting and cause undesired results. Many people (especially in a studio setting where the lighting doesn't change) will manually set their white balance to match their studio so they know they will get consistant results.