What is the purpose of setting maximum spacing of piles?

One of the factors that affect the distribution of loads from the structures to each pile is the assumption of flexibility of the pile caps in design. A pile cap can be modeled as a flexible or a rigid element based on their relative stiffness. For the pile cap to be assumed as rigid the stiffness of pile cap is infinite relative to that of pile/soil system and the deformations within the cap are not considered owing to its rigidity. On the other hand, for the pile cap to be designed as flexible, internal deformations of pile cap would occur.

In some design guidelines, maximum spacing of piles is specified to limit the length between adjacent piles so that the assumption of rigid pile cap can be justified.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.