What can I use it for?

That depends on who you are and what you write about. Doctors, medical transcriptionists, lawyers, tech support operators and professional writers are amongst the most prominent types of users who have bought a TypeIt4Me user licence so far.
In my own case, I find it extremely useful for typing the current date and my return address at the top of a letter and for my standard end of letter salutation, for which I type 'ys' and get:

Yours sincerely,

Riccardo Ettore.

I also use it to type standard phrases when providing email support for my shareware.

Abbreviations don't have to be shorter than the text they type. For example, I defined 'tilde' (5 letters long) to 'expand' to a single character ('~'), which I often need in C programs, yet always manages to disappear from the keyboard when I most need it! And eur 'expands' to €.

By defining a set of mnemonic abbreviations for the most frequently used words, you can type faster yet more accurately with less effort. On page 387 of its May 1990 issue, MacWorld has this to say about a commercial program that does the same - at nearly twice the price: "inexpensive;saves keystrokes;has potential to help RSI sufferers".

For example, "u r rh ab teh cgs" could type out "you are right about the changes". (Assuming that you have defined u=you, r=are, rh=right, ab=about, teh=the and cgs=changes and that you defined the space bar as one of your expansion triggers.)

Another application of TypeIt4Me should become apparent if you look closely at the previous example:
Notice how the common mistake of typing "teh" instead of "the" gets automatically corrected as would "adn" for "and", plus any others that you can think of. So assuming, of course, that you create the appropriate abbreviation/clipping pairs first, TypeIt4Me can help you eliminate your most common typos on the fly.

TypeIt4Me is also very useful for entering repetitive data in a database file or spreadsheet.

Or to log into various systems by typing an abbreviation that expands to your user ID then tabs to the next field and pastes your password.

Naturally, you will probably find your own, novel uses for this program. If you can spare the time, I would definitely appreciate hearing how you use it and share your tips and hints with the other users.

The bottomline is: anyone who types anything at all on their Mac can benefit from TypeIt4Me.