The three - Kavacha, Argala and Keelakam are considered integral part of the Chandipaath or Devi Mahatmyam. This is because the sadhana is designed along the guidelines of ShaktA tradition.
The core 700 verses of the main text can awaken Shakti in a very furious and volatile manner which can be too overwhelming or at times dangerous or raw for a seeker to handle. Hence the need for Kavacha - in order to protect against any disturbance. Another thing that every sincere seeker will face are interaction with various forces whose only aim of existence - their own peculiar dharma - is to break the sadhana at any cost. They can use weaknesses within and without the seeker to serve their own ends.
Then the Argala - bolt - asking for a particular form of intervention of that Shakti in the seeker's life. Remember that our scriptures talk not only of Moksha but also Dharma, Artha and Kama. The untempered Shakti of the Chandipaath can be too daunting unless one has gone beyond ALL desires. But since most are yet to reach that stage, hence the Argala is a process of asking that Powerful and Divine Shakti to work in a specific manner to help the seeker. Shakti - uncontrolled - is like a sword that can cut through anything and everything, good or bad. Dharma/adharma makes little difference. Hence the best Shaktiman is one who is in perfect detachment, at all times, unwavering steadiness of mind and chitta.
Then comes the Keelakam. This is a crucial part of the Tantric process. The seeker is trying to fix the energy of the mantras into a specific areas instead of letting it scatter. All these are necessary at the initial stages of awakening Shakti. Unless the Shakti is tied to a place, it will take a huge amount of time to build up to a reasonable strength. So that is why these three were/are considered crucial part of the Chandipaath.
(to be contd.....)