Unicode is a character set supported across many commonly used software applications and operating systems. For example, many popular web browser, e-mail, and word processing applications support Unicode. Operating systems that support Unicode include Solaris Operating Environment, Linux, Microsoft Windows 2000, and Apple's Mac OS X. Applications that support Unicode are often capable of displaying multiple languages and scripts within the same document. Each encoding has advantages and drawbacks. However, one encoding in particular has gained widespread acceptance. That encoding is UTF-8.
Well basically because UTF-8 includes nearly all characters from all languages. This makes easier for developers to ensure what multi language text will be displayed correctly every time.