Chairs are usually the only real prop, which is present from the outset or at the edge of the stage and actually used when needed for games, i.e "Busy" is. The reason is probably the fact that sitting on a fictitious chair with moderate force for a long time is not possible in mime.
Chairs are often much too quickly used as a real prop. Especially for beginners, it is so that they quickly sat down on these chairs and the physical game is neglected. There is then to land in a pure speech scene the danger. That is why chairs should only be used when it is absolutely necessary.