There are actually two different kinds of titles. Threads have titles, and messages have titles. The message title is, by default, the same as the thread title, unless you change it at the time you post your message. 99.99% of the time, people don't bother to change it. Like you, they probably don't know they can change it, or don't see any point in changing it.
The message titles—also known as "subject lines"—are used mainly by the JoeClones, who have the power to change them after the message has been posted. If you post the lyrics to a song, and a JoeClone happens to see it, he or she will probably change the message title so that it contains the song title. This is very useful because the thread title often doesn't contain the song title. Many people request lyrics without knowing the correct song title. Sometimes several related songs with different titles are posted in one thread.
Message titles can be searched. Go to the drop-down menu at the very top of the page—the one that at first says "Quick Links"—and select "Old Adv. Forum Search." On the new page that comes up, type a song title into in the "Subject" box and click "Search." It will bring up all the messages that have that in the title, even if the message title is different from the thread title.