Everything in this creation, including you and I, is a combination of two principles: Person and personality. We find the play of these principles wherever we look in the relative universe. A glowing bulb is the result of the union of its filament and electricity, neither of which can illuminate a hall on its own. Light is only possible when these two factors combine. Electricity manifests as light through the bulb.
The universe is also a creation of the union of Person and personality, known as purusa and prakrti in Sanskrit. Person is the Consciousness, and personality is the matter. Consciousness manifesting through matter becomes creation, life. Consciousness and matter are relative manifestations of brahman. The Person, Consciousness is usually looked upon as the male principle and personality, matter is looked upon as the female principle. The union of these two results in creation.
Hence, gods also have consorts. Nothing can be accomplished without the involvement of the two principles. Even as I speak now, the Person, Consciousness, is functioning through the organ of speech. Action also can take place only when there is a union of Person and personality. Brahma, the creator, is the purusa aspect as far as the creation is concerned. Functioning through his consort Sarasvati he creates. Lord Visnu functioning through Laksmi sustains and Rudra/ Siva functioning through Parvati/Sakti/Durga dissolves or destroys. Of course there are no strict compartments because these names of gods are also ultimately relative.
Lord Visnu can also be looked upon as the ultimate truth.
In one form such as Visnu we can invoke saguna-brahma as well as nirguna-brahman. When we invoke the saguna aspect, brahman with attributes, then we call him protector and as nirguna He is Satchitananda, beyond all attributes, names and forms. It is the same with Lord Siva. In the corresponding puranas we find that respective Gods are presented as creator, sustainer and dissolver/ destroyer. So there are no hard set rules. Generally however, Brahma is looked upon as creator, Visnu as preserver and Siva /Rudra as destroyer.
The idea of the consort is that without Sakti, prakrti, the female principle, the male principle cannot accomplish anything. Action or creation can only take place when both principles are involved. Therefore there is a consort associated with every god, and the nature of the consort will correspond with the function to be performed. For Brahmé to create, knowledge is required. Sarasvati is the goddess of knowledge. So they are associated with each other. For preserving and maintaining, resources are required. Laksmi, the goddess of wealth and prosperity is the consort of Lord Visnu. Power is required for dissolution. Parvati/Durga is the goddess of power with which Lord Rudra/ Siva destroys.
Swami Viditatmanand Saraswati