To generate a star in any text document or browser-based text window, holding the ALT key while typing 9733 will give a solid star. The code 9734 will give an open star with no shading. However, this is just one application for alternative characters.
There are tens of thousands of different special characters that can be generated using the correct unicode, or hexidecimal number, in combination with the ALT key. In most cases, these characters are reserved for either foreign language practices or for internal coding. However, some people have begun using these special characters in everyday applications. This may be a post to their social media account, an email to a friend or just a clever way of creating a unique user ID. While these may not be the primary reason behind ALT codes being input into a computer's programming, they are gradually becoming more common place in many settings. A star in an email adds flair, while a social media post with subscript, superscript or appropriately placed accent marks might be more memorable to readers. While there aren't necessarily any conventions that restrict use of these characters, the average person might want to use caution before liberally sprinkling them throughout their daily writing.