To provide students with a means of checking their progress at each and every question. Having answers provided for worksheet questions does not eliminate the questions' challenge, because the answer itself is not the ultimate goal of the question, anyway. Rather, the learning objective in each case is the method of solution, which is purposefully omitted from most answers.
As someone who learned a lot of his electronics knowledge from working through the problems at the end of textbook chapters, I appreciate having answers provided to every question. It seemed to me that many of the best questions I encountered in textbooks -- the questions I really desired to know the answer to in order to check my work -- were not supplied with answers. This was aggravating to me because I was not enrolled in a class at the time and had no one to turn to for verification of my learning.
Some may believe that by giving away answers to each and every question, students will not learn as much. The belief is that only some (or none) of the answers should be given away to students, lest accountability for research and independent thinking be lost. This concern may be justified in the traditional application of practice problems, but not in the context in which these worksheets are designed to be used.
The purpose of these worksheets is not to teach students the "right" answers to certain questions, but rather to teach them how to research and how to think for themselves. In other words, obtaining the answer to a question is not the end-goal of the question at all. Rather, the end-goal is the development of research and problem-solving ability. It is during discussion time, when the instructor challenges students to explain how they arrived at their answers, that accountability for learning takes place. It is here where each student interacts Socratically with the instructor to explain how they found the information, how they solved the problem, and is challenged to reason through deeper levels of the same problem, that the learning process becomes complete. The purpose of the worksheets is simply to prepare students for a fruitful discussion experience.
This is why I caution against using these worksheets as mere supplements to traditional instruction. Of course, there is no harm in doing that, but there will be little benefit. The questions contained in these worksheets, and their respective answers, are merely catalysts for learning. Self-motivated students may learn a lot from working through the questions, but the average student will not.
Some instructors may find it beneficial to base a portion of the course grade on the quality of participation exhibited during discussions. This provides a measure of external motivation for each student to work through the worksheet questions as they were designed to be used. The necessity of this approach varies with the maturity of the students. I have found that most of my college-level students, especially the adults, need no such motivation to fully engage in the worksheets. In fact, toward the end of my first year of teaching this way, I was amazed to find that many of my students purposely avoided reading the answers, so as to challenge themselves as much as they could! Of course, their attitude was not like this at the beginning of the course, but it goes to show that teaching in this manner helps to develop a genuine respect for learning.
But what about students who absolutely refuse to do their own research or work through problems independently, who choose to merely repeat what the provided answer says when it comes their turn to share in discussion? Is it detrimental to reveal the answers to students like this? To this I say: No! The root problem with such a student is an utter lack of desire to learn: a problem that cannot be remedied by any change in worksheet content or teaching technique. A student like this will learn no more following this method of instruction than if they passively listened to a lecture, which is to say, not much. One can only hope that such students will catch some of the enthusiasm of their classmates.