There are many reasons for taking lecture notes. Making yourself take notes forces you to listen carefully and test your understanding of the material. When you are reviewing, notes provide a gauge to what is important in the text. Personal notes are usually easier to remember than the text. The writing down of important points helps you to remember them even before you have studied the material formally.
Instructors usually give clues to what is important to take down.
Summaries given at the end of class. Reviews given at the beginning of class. Never use a phrase when you can use a word. Use abbreviations and symbols, but be consistent. Put most notes in your own words.