What is the purpose of kirtan in Sikhism?

Kirtan with musical instruments was started by Guru Nanak and followed up by other Gurus. Such type of Kirtan is played in all Gurdwaras. Guru Granth Sahib is also written in Ragas. Mention of Kirtan in Guru Granth Sahib is to remember the Virtues of the God every moment of life.
ਓਅੰਕਾਰਿ ਏਕ ਧੁਨਿ ਏਕੈ ਏਕੈ ਰਾਗੁ ਅਲਾਪੈ ॥ ਏਕਾ ਦੇਸੀ ਏਕੁ ਦਿਖਾਵੈ ਏਕੋ ਰਹਿਆ ਬਿਆਪੈ ॥ AGGP 885
Onkar Ek Dhun Ekai Ekai Rag Alapai || Eka Desi Ek Dikhavai Eko Rehia Biapai ||
ਏਕਾ ਸੁਰਤਿ ਏਕਾ ਹੀ ਸੇਵਾ ਏਕੋ ਗੁਰ ਤੇ ਜਾਪੈ ॥੧॥ ਭਲੋ ਭਲੋ ਰੇ ਕੀਰਤਨੀਆ ਰਾਮ ਰਮਾ ਰਾਮਾ ਗੁਨ ਗਾਉ ॥ਛੋਡਿ ਮਾਇਆ ਕੇ ਧੰਧ ਸੁਆਉ ॥੧॥ ਰਹਾਉ ॥
Eka Surat Eka Hi Seva Eko Gur Te Japai ||1|| Bhalo Bhalo Re Kiratania ||Ram Rma Rama Gunn Gao ||Chod Maya Ke Dhandh Suao ||1|| Rehao ||
ਪੰਚ ਬਜਿਤ੍ਰ ਕਰੇ ਸੰਤੋਖਾ ਸਾਤ ਸੁਰਾ ਲੈ ਚਾਲੈ ॥ ਬਾਜਾ ਮਾਣੁ ਤਾਣੁ ਤਜਿ ਤਾਨਾ ਪਾਉ ਨ ਬੀਗਾ ਘਾਲੈ ॥ ਫੇਰੀ ਫੇਰੁ ਨ ਹੋਵੈ ਕਬ ਹੀ ਏਕੁ ਸਬਦੁ ਬੰਧਿ ਪਾਲੈ ॥੨॥
Panch Bajitr Kare Santokha Sat Sura Lai Chalai|| Baja Maan Taan Taj Tana Pao N Biga Ghalai||Feri Fer N Hovai Kab Hi Ek Sabad Bandh Palai ||
ਨਾਰਦੀ ਨਰਹਰ ਜਾਣਿ ਹਦੂਰੇ ॥ਘੂੰਘਰ ਖੜਕੁ ਤਿਆਗਿ ਵਿਸੂਰੇ ॥ਸਹਜ ਅਨੰਦ ਦਿਖਾਵੈ ਭਾਵੈ ॥ਏਹੁ ਨਿਰਤਿਕਾਰੀ ਜਨਮਿ ਨ ਆਵੈ ॥੩॥
Nardi Narehar Jaan Hadure || Ghunghar Kharrak Tiag Visure || Sehaj Anand Dhikhavai Bhavai || Eh Niratkari Janam N Avai ||3||
ਜੇ ਕੋ ਅਪਨੇ ਠਾਕੁਰ ਭਾਵੈ ॥ ਕੋਟਿ ਮਧਿ ਏਹੁ ਕੀਰਤਨੁ ਗਾਵੈ ॥ ਸਾਧਸੰਗਤਿ ਕੀ ਜਾਵਉ ਟੇਕ ॥ ਕਹੁ ਨਾਨਕ ਤਿਸੁ ਕੀਰਤਨੁ ਏਕ ॥੪॥੮॥
Je Ko Apane Thakur Bhavai || Kot Madh Eh Kiratan Gavai || Sadhsangat Ki Javao Tek || Kahu Nanak Tis Kirtan Ek ||4||8||
Blessed is such a Kirtania, who renounces the entanglements and pursuits of Maya and sings the Glorious Praises of the God. Such a person have faith in One Universal Creator; sings the tune of (Talks Of) the One God. Mind (Mann) remains attuned to the One God and preaches the same to others. Such a person centers consciousness on the One God, and serves only the One God, who is known through the enlightenment of the Guru. Such a person makes the five virtues, five musical instruments; earning honestly is the musical seven notes, is always contented. The harmonium such a person plays is the renunciation of pride and power. Does not let mind (Mann) get entangled in Maya, is prelude to song. Such a person always keeps spiritual knowledge and wisdom in the mind (Mann) and so does not enter the cycle of spiritual birth and death. To play like Narad, is to know that the God is ever-present. The tinkling of the ankle bells is the shedding of sorrows and worries. Dramatic gestures of acting are celestial bliss.Such a dancer does not enter ycle of spiritual birth and death again and again.
For more information listen to my you tube on Kirtan. For more such topics you can read my book Gurmat Doctrines (Part1) and (Part2) available on amazon.com