Spiritual significance of Pancha muga Kuthu Vilakku - Meaning and Why?

For the benefit of non Tamilians, Kuthu Vilakku is an oil lamp, lighted in pooja room in front of the deities in every home.

One perspective answer:
God is in the form of light In side of U to drive out the darkness of ignorance and to give ultimate happiness. So, Kuthu Vilakku is lighten to drive out all bad things in and around you and to bring you all good things and happiness.This represent god himself come and present before you in the form of light.The number of face( MUGAM) represent the particular deity and the letters of Beja Mandram.

Panchamugam means five face or five split-ed light represents lord Siva the deity who has five face.The basic chanting( BEEJA MANDRAM) is NA MA SI VA YA five letters.When U light Pancha Muga Kuthu vilakku and chanting AUM SI VA YA NA MA then God Siva come there to bless U and the people there . Grant all your prayers .
It is told in the "Nandesar negandu 300 padal 24 as" DEEBAM ENDRA PANCHA ARTHUKU
INEDU DEEPAM:". With blessings of SIVA..

Another perspective::
Worship of a deity is a subject of faith for all of us.

The lamp has its own special place in Sanatan Vaidik Hindu Dharma. It is a form and a symbol of Tej (Absolute fire principle); the lamp leads us from darkness towards light. It burns only to give message of peace and light to the man. This is its greatness. Agni Puran clearly states that only oil or Ghee (clarified butter) be used in the lamp meant for puja and no other combustible substance. According to the Science of spirituality the lamp with clarified butter is more sattvik (spiritually pure) as compared to lamp in which oil is used.
According to yogic path of Kundalini Yog there are seven principle chakras or energy centers in the human being. These chakras influence practically every aspect of human being including physical body, mind and intellect. The oil lamp is effective in purification of Muladhar and Swadhishthan Chakra only to certain extent but the ghee lamp purifies Manipur and Anahat chakras to a significant extent.
Just as there are seven chakras in the human body, there are paths for the flow of vital energy (chetana). These are called as Nadis or channels. The three principle nadis are Chandra nadi (Moon channel), Surya nadi (Sun channel) and Sushumna nadi.When Chandra nadi is active the person perceives coolness. The activation of Surya nadi imparts energy to the person. The Sushumna nadi is activated when the person starts progressing spiritually. The oil lamp activates the Surya nadi of the person while ghee lamp activates only that nadi which is essential to the worshipper in a particular action.
There are other varieties of the lamps also such as lamp with a single wick and lamp called niranjan wherein five wicks are used. The Niranjan with five wicks is symbolic of duality that is the manifest energy of a deity; whereas the lamp with a single wick attracts sattvik frequencies the lamp with five wicks attracts waves with destroyer property and dominant in raja component.

The niranjan with five wicks denotes the relation of Panchpran (five vital air principles) with the Atmajyoti (flame of soul). Niranjan is used for waving Pancharati. Every single flame of niranjan is symbolic of Atmajyoti. Pancharti means invocation of God with the help of panchparanas.While performing Pancharti we should have such a spiritual emotion that the Atmajyoti is kindled in me with the help of five panchaprans present in the body and I am performing the arti with such flame.