I would imagine the original purpose was to encourage people to marry and have families. Article 41 of the constitution states:
"1° The State recognises the Family as the natural primary and fundamental unit group of Society, and as a moral institution possessing inalienable and imprescriptible rights, antecedent and superior to all positive law.
2° The State, therefore, guarantees to protect the Family in its constitution and authority, as the necessary basis of social order and as indispensable to the welfare of the Nation and the State.
2. 1° In particular, the State recognises that by her life within the home, woman gives to the State a support without which the common good cannot be achieved.
2° The State shall, therefore, endeavour to ensure that mothers shall not be obliged by economic necessity to engage in labour to the neglect of their duties in the home."
The tax system recognises and facilitates this "unit" of family by allowing joint assessment to tax, and an element of transferability of tax credits and rate bands. It facilitates parents who may wish to reduce or refrain from participation in the general workforce in order to maintain the family unit.