You can use a colon to mean "that is," as well. She's Mexican: Half Spanish and half Native American.
It doesn't mean "that is" there, does it? Unless all Mexicans are half Spanish and half Native American.
the first question you should ask is WHY? then ask the origin of how they came about and I am hazarding it a guess is that they came about to suit the keyboard/typrwriter/norse code. if you research the bible for example or a very old book from past years or look at the way hieroglyphics writing you would not find on single punctuation apart from the odd full stop or comma. so these darn {;} {:} are just useless when it comes towriting in general because pen came first and so you should follow suit.
