Grammarians and linguists are familiar with the idea of a function of the ‘absence’ of morphemes which is currently called “zero”. Western linguists beginning with de Saussure's work of 1879 have often postulated the existence of the so-called zero-morphemes where the actual perceptible linguistic form does not match its relevant semantic and syntactic content (see T. Pontillo 2002, p.559ff.). They resorted to this device on the basis of a significant opposition pointed out between comparable morphological structures.
As focused by Al-George (Al-George 1967, p.121), on the other hand, the Indian linguistic zero is not a mere device, adopted for a descriptive purpose. It rather seems to represent “the consequence of a definite philosophy of form”, namely “the category which exists though not embodied in a concrete form, suspended as a pure virtuality at the border between existence and non-existence”.
A more general problem is: How can an absent element perform a function notwithstanding its absence? How comes that an effect can be grasped in absence of its cause?
On the latter problem, see here (on tantra and prasaṅga as a possible answer).