Why we use 12mm bars as chair in slabs work?

@Viraj‍ - can you answer this? 

Disclaiming any domain knowledge, I understand that the chairs are used to maintain the gap in the reinforcement of slabs. They also prevent collapse of the bars when work starts in making the slab. (People walking on the reinforcement and such). Since 10 mm bars are normally used in the reinforcement, chairs using 12 mm bars at specified intervals are used (at least in the Indian context) for this.

This may help some: