The triple functions
The Lord who began to create thus, continues with his work as Brahma who is full of raja-s. He guards the created world as Vishnu who is full of sattva. Devolving on others the work of creation and destruction that are frowned upon by everyone, He takes charge of sustenance which is hailed by all. The compassionate Krishna becomes tamas filled Rudra in the end and devours all that He has created. At that time there is only water everywhere, and in the midst of it, He rests in yogic sleep, guarding the world within Himself.
Apt smiles
Imagine someone, who possesses a highly valued gem, sojourning in a forest notorious for high-way robbers. It gets dark. He puts the gem into his mouth for sagety. Once he comes out of the forest, he spits it out and carries it in his usual manner. In the same way, the compassionate Lord minimizes the entires world without any harm coming to it, and devours it to guard it. ONce the danger of the deluge is over, He re-creates it as a recognizable unit and is happy. A farmer tills his land and grows crops. Once it grows 
well and the yield is taken, he mows down the field and raises another crop again. Our ancients have spoken of the Lord and His work of createion, sustenance and dissolution in the same manner and likened Him appropriately to a farmer, hari krisheevala. The Primal Lord is this Vishnu who creates, devours and brings out a well-constructed world. As the creator He is known to be the four-faced Brahma, as the sustainer He is hailed as Vishnu and as the destroyer He is referred to as Siva or Rudra.
Lord Vishnu is both Brahma and Siva
The Supreme Lord is the creator. That shich is created is also the Lord. It is Krishna who guards the world. It is Krishna who is guarded too. Ananta destroys and what is destroyed is Ananta! We speak in these terms because the world and Brahman are like body and soul. Such is the definition arrifed at by our ancients. This is the significance of our referring to Brahma and Rudra also as Vishnu. He is their soul directing their works. This is explained by Prasara to Maitreya throught the following coulet:
Sa eva srijayah sa cha sargakartaa
Sa eva paatyatti cha paalyate cha
Brahmaadyavasthabhir asesha-moortih
Vishnur varishthah varado varenyah
“He himself is created; He is the creator himself. He alone protets and swallows; He Himself is protected. The One assuming all the forms is Vishnu, the supreme, the granter of booms and the covetable One. All this He does, thorugh different states of Brahmaa and others.” Thus are answered questions regarding the state of the world raised earlier, its reappearance, its primal cause and the soul of the world.
Source
Sri Nrisimha Priya
