White watercolor paint is a freaking waste of time and space. I have some. I don't use it. Why? It doesn't work!
In other painting mediums, white is used to add or restore highlights. But watercolor is transparent, so trying to put white watercolor over darker colors simply doesn't do much! You can still see the layers underneath! Thus: pointless.
If you want to add opaque white as a finishing touch, you gotta go with gouache or acrylic. Or--as many watercolor snobs will tell you--you gotta keep the white of the paper for your whites. Now I'm not a tyrant--I don't think you have to follow that rule or you're "not A REAL watercolor artist" (huge eye roll here) and I use acrylic or gouache for white in the top layer if I want.
I'm also not saying no one could ever find a use for white watercolor paint! Some people use it to make fully transparent colors more opaque, and if that's your jam, go forth and enjoy it! Break whatever "rules" "the man" is trying to use to squash your innate love of creating art. I'm just gonna stay over here giving major judgy eye to any set of watercolor paints that include this practically useless color.