Blocks create different physical levels. You can sit and stand on them. Sometimes traditional blocks of wood are fine. Sometimes you want to perform the same purposes with something more unusual such as a chair or a bench. Blocks allow more opportunities for creative blocking.
Blocks can help enhance the story.
They generate interest. In Chicago, our students saw one of the casts walking down the hallway with garbage cans and wanted to follow them and see their show. It's fun to create that kind of interest. We bought the trunk to be our box. It is important to note that blocks aren't necessary.