Sometimes just a slightly wrong choice of words can create a real misunderstanding, or provoke hours of wasteful argument. Sometimes people would rather argue about who said what and how than ever try to discuss something meaningful and enjoy each others company. This tool is often used by those who never want to be wrong or "lose an argument." All it really does is put up a block between the two of you and prevent any genuine intercourse from occurring.
Philosophy of language
At the cutting edge of understanding, we try to understand how words can have "meaning," and wonder how we manage to communicate at all. This study also gives us some tools for determining the validity behind the assumptions in certain questions, via the understanding of how aspects of language are sometimes twisted or used poorly. My favorite example is "what is the meaning of life?" The idea of "meaning" comes from our use of words, and also to an extent our actions. "What do you mean by that?" is a perfectly legitimate question, since words are supposed to have meaning. Transmuting the idea of meaning to the concept of how we live our lives can make the question vague, and easy to try to answer the wrong way.
Concern for feelings
Our concern for others in our use of words can be a double edged sword. On the one hand, we would like to make sure our meaning is clear, avoid misunderstandings, and where possible, not cause unnecessary anguish. On the other hand, I have found that many people would rather simply avoid "saying something mean" when it is the right thing to say - the true and harsh words that are necessary at times. We do not want to think of ourselves as harsh or cruel, but if the expression of what we feel will take this form, sometimes we must have the courage to use it.
Plain old PC crap
"Political Correctness" has its roots in the desire to reduce the oppressive power of bad habits in language. We really must teach ourselves to remove the prejudiced orthodoxies from our casual language - to not ask "what is his name" when someone mentions their doctor or lawyer (or plumber!), to refer to the person running a meeting as the "chair" and not the "chairman." To at least be aware of the loaded language we have inherited and how it can limit peoples perspective on who they can be. But... when it reaches the point of censorship and prior restraint, not being able to say certain words without being jumped on, not being able to even discuss certain ideas... there is something wrong, the process has gone from an attempt to open the world up and make it better for all, to simple autocratic suppression.