There's a lot you can do with your preschooler using this tool to teach them the ABCs. You could:
As a creative exercise, you could generate a couple letters and come up with a word or sentence that includes them. For example, if the 3 letters are "H", "A", "F", a sentence could be "Horses run fast."
There are other generators available like this. You might want something more specific, like actual random words. For something more techincal, there's a random string generator. Also, don't use this for a password, use the random password generator.