It's not absolutely essential but I do highly recommend it, especially if it's your first time making caramel. Without using a thermometer, you can tell when the caramel is ready when it starts to pull away from the sides of the pot. There is a risk when doing it this way though ; the caramel may end up a little to soft or too hard and brittle. Using a thermometer lets you measure the exact temperature which will ensure the caramel is the correct consistency.