Risk reduction I.e. NPA reduction.
E.g. ICICI offers as many cards as one want if one has 5l limit & has 6 cards.
& if plans to default it can cause 30l NPA to bank but in case of shared it's only 5l.
Other aspect limit is assigned basis on credit worthiness which is just 5l not 30l.
If one's credit worthiness allows him 30l limit,he can get 30l limit on single card.