How can a computer know who I am?
It doesn´t. But with a good amount of text it can make a pretty good guess. In addition, we all have different roles in different situations. By studying how people write, we can get an glimpse of what is on the mind of the person and what is characteristic of the person - or, as in the case of a blog - what is typical of the role/persona used when writing that blog. A person can have several blogs - and often have different roles for the various blogs - perhaps as a way to live out more sides of themselves.
What does a text say about the writer?
A lot more than most people might think. But it does NOT tell you who you are! A text is more to be likened with the clothes you´re wearing. The words and expressions we use, especially when we write for our own pleasure, tell about if we are more inclined to logical reasoning, personal values, fantasies or the reality for instance.
A personal text can be likened with the clothes we wear. We all tend to have clothes that we feel the most comfortable with - that we feel communicate something important about ourselves. However, almost all people accept tp dress differently in different situations - like at work, when we´re on vacation or when we want to make a special impression to someone else. Reading a lot of text over a period of time is therefore more interesting than just reading a few that might be written by a special purpose in a certain situation.