In addition to the textbooks found in bookstores, individual worksheets--including those that you make or those that you find online--can be the difference between a successful class and one that fails to achieve expectations. After spending years teaching students textbook after textbook, many EFL teachers begin to feel exasperated at their students' lack of progress. This is understandable given the relative uniformity of many texts and the fact that a textbook written for one culture may have limited suitability for another culture. In this context, ESL worksheets can often be the difference between a class that progresses rapidly and a class that becomes frustrated and falls behind.
Granted, teachers may need to spend some of their free time to develop and print worksheets, but this hard work will certainly pay dividends, both in terms of the overall enjoyment level of the class and students' abilities to grasp the material. Best of all, worksheets can be used in combination with any textbook series. For example, if students encounter particularly difficult vocabulary words or grammar points in a chapter, the teacher can make an introductory worksheet for the following class in order to ensure that everyone is on the "same page".
Moreover, these worksheets help create more structure for the children. For example, each class can begin with a warm-up, progress to the textbook's standard lesson, then feature an English worksheet, after which the class can be ended with a review session. Children respond well to structures such as this, and the chance to do a worksheet either by themselves or in groups gives them a welcome respite from the tedious and repetitive nature of most lecture-based classes. Teachers should thus look at worksheets as more than simply assignments, and as activities that can inject joy into each class!