You have some interactive demonstration tool (eg. like an applet or interactive application) and only one computer or projector to use for the entire class. Another example might be a problem involving the use of algebra tiles that are visible underneath a document camera for someone to manipulate.
You want all students to record observations, noticings, wonderings, generalizations, or respond to prompts based on the interactive demonstration.
There is a set of steps you want everyone to be able to follow without losing anyone in the audience. Eg. You are introducing students to using some feature on their calculator.