It became common due to the small number of characters allowed in a text message and the small keyboards on mobile phones.
Txt includes some of the old standbys of using letters (r, u, b, c) as a rebus in place of entire words (such as "are", "you", "be" and "see"). It also retains the use of numerals to shorten text: 2 can do double-duty for "to" and "too", while 4 represents the word "for" as well as the syllable "for" at the beginning of words (4get). "Before" becomes "b4" turning both symbols into singular characters. The numeral 8 represents the "8" sound at any place in a word: sk8, gr8, vac8ion, sk8er, hesit8, w8ress, 8ypical.