A tautology is a statement that states itself. For instance, that last statement is somewhat of a tautology, since a tautology is a tautology, ie a statement where one says that A is A.
For some reason, the one example everyone use to demonstrate what a tautology is, is that an unmarried man is a bachelor. One can clearly see that "an unmarried man" and "a bachelor" amounts to the same thing - A is A. Saying A twice does not amount to A being A twice as much as if it had only been said once - the relation between A and A remains identical, as it were.
Philosophers love these kinds of relations. They don't have to be proven empirically - they just follow from a proper understanding of definitions. It's neat, orderly and proper. One can depend on tautologies and the intricate relations of implicit definitions that follows from them.
Definitions are solid things. Once they are penned, they remain.
You know something that is not neat, orderly and proper? Rhetorical situations. They tend to be the opposite - messy, disorderly and improper. And, moreover, they tend to not give too much credence to what the implicit logic of grammar and language has to say behind the scenes - there's too much going on to really give the participants time to ponder exactly what makes "it" rain in the sentence "it is raining".
There is, in fact, enough going on in a single social moment to fill several descriptive pages of descriptions. Just to mention a few factors going on: who's talking, to whom, about what, from what position, in what role, for what reason, with what credibility, with how much ethos, with - the list goes on for quite some time. And this is just for one singular moment - stretch it out in time, and it will become infinitely more messy and hard to relate.
This is important. This is the difference between philosophers and rhetoricians. The former act and perform in the eternalities of the good, true and beautiful. The latter act in the fleeting moment of the moment - and they act through speech acts.
Which brings us to tautologies. They don't add anything new semantically to the context - that bachelor is still unmarried, much to his chagrin. They do however add something social to the context - merely by the fact that the speaker speaks.
It does not matter what's being said. The fact that something is being said at all carries with it an enormous amount of implications - even if it's just stating the obvious.
It follows, then, that there is a wide array of uses for tautologies. An obvious one being that you mark yourself as a participant of the discussion - you are talking, and therefore take part of/in the interaction as a talker.
I'm not going to bore you with a list of specific situations. The point I'm trying to get across here is that there is a difference between what is semantic and what is social, and that the one sometimes has little or no bearing on the other. And that you will want to ponder this difference as you go about your daily life taking part of various social situations. Is it more important that you say the right specific thing, or that you're saying something in general in the right context?
A is A. And then it becomes an excuse to say something else.
I leave you to your own devices in determining what that 'something else' might be. -