Explanation:

Demonstrations offer useful opportunities for purposeful participation in learning that can enhance understanding of the concepts given in the textbook. Leading science educators make the point that there is always a place for an ‘interesting, sometimes unforgettable, demonstration that may form an important episode in a student’s learning’ (Wellington and Ireson, 2012).
A demonstration provides a shared experience that allows you to focus your students on particular aspects that they might otherwise miss. You can use demonstrations to support your explanations and scaffold your students’ understanding. 

There are other good reasons why students cannot do the practical work themselves. For example, with large numbers of students, the time, space and resources required may not be available. As a teacher, you have to use your professional judgment to decide when a demonstration is appropriate. You need to justify that decision in terms of your students’ learning, rather than on what is easiest for you as the teacher. Not all processes and phenomena lend themselves to classroom demonstration; some may be too complicated or lengthy to be practical in the classroom environment.