Witnesses are not limited to "eyewitnesses." Witnesses may be called because they actually saw or heard a crime occur, but they also may know something about a piece of evidence, or may possess information that contradicts another witness' testimony. You may not think that what you know about the case is significant; however, small pieces of information are often required to determine what really happened. If you want to know why you are testifying in a particular case, ask the prosecutor or your victim/witness assistant; there is probably a good reason.
Please keep in mind that your presence and willingness to testify may be the deciding factor in determining what will be done in the case. Many defendants often hope that you or other witnesses will not show up. Sometimes, your mere presence at the courthouse before the trial may be enough for the defendant to plead guilty.