Awarded as the best online publication by CIDC
In some codes, a minimum distance between bars is specified to allow for sufficient space to accommodate internal vibrators during compaction.
On the other hand, the restriction of maximum bar spacing is mainly for controlling crack width. For a given area of tension steel areas, the distribution of steel reinforcement affects the pattern of crack formation. It is preferable to have smaller bars at closer spacing rather than larger bars at larger spacing to be effective in controlling cracks. Hence, the limitation of bar spacing beyond a certain value (i.e. maximum distance between bars) aims at better control of crack widths.
If you have a query, you can ask a question here.