There is no definite answer to this question.
Shirdi is a small village in Maharashtra, when Baba was first seen as a strange person in a marriage procession at the village. The Poojari solemnising the wedding ceremony spotted and addressed Him “Aao Sai”. The meaning of the word stands for a pious person, an elderly person, a saviour etc., At that time, the villagers have not been recognising Baba. During next visit, Baba made Shirdi as his permanent abode. The locals called on him, considering Baba as a person who could cure their ailments. Baba used to advise remedial measures for curing their illnesses.
He used to burn fire, close to Him and ash from it was given to people who visit Him which He called “Udhi”, Holy Ash. Shri Sai Baba thus became known to the villages. He possessed immense knowledge of the Holy Books of all the religions such as the Vedas, Quran, Bhagavad Geeta, Bible and Yoga Sutras. He was also a linguist and known to different sectarians of the society and everyone irrespective of class, creed caste or religion gathered around him for peace and solace. There were Hindus, Muslims, Parsis, Christians all under one roof in front of Him. He was compassionate and loving to all beings, including animals and plants.

The miracle of Shirdi Sai Baba is unbound and a testimony to the entire world. He is a Sadguru in the garb of a Fakir, and many consider Him as "Sai Maharaj" and also a "Raja Rishi" serving the poor and the rich alike to mitigate their sorrows and sufferings. His life and works are very precious to us and stands as beacon light to all humanity. He is a King as well as a Servant, at the same instant. He is also fearsome for reasons known to him only.

The Saintly and Holy offers things which stands precious to their devotees and people. Our Sai Baba of Shirdi offers Udhi or Sacred Ash as 'Prasad'. Devotees dakshina the offering in cash made to Baba(dakshina), is spent by Him to buy fire wood and keep the fire continuously burning till today. The ash from the fire is taken by Baba and offered to devotees and blesses them.

The holy ash, Udhi, signifies Vairagya (dispassion and detachmen). Vairagya is born of Viveka or discrimination between eternal and non-eternal, real and unreal, essence and non-essence, which comes through selfless service and devotion. Thus, it supports one with spiritual strength. That’s why Baba gives the Udhi as Prasad.

In fact, He guides everyone to the ultimate realisation of “who you are “, to attain Oneness with Omnipotent, Omniscient and Omnipresent, Sai, God, Bhagvan, Allah, Father in Heaven, Jehovah, Ahuramazda, in whatever name one calls Him. All the name stands for Sai, which is Pure Awarness of the Self or Atmabodham.

The offerings of the Devotees signify surrendering of ego to our Master who reduces it to ash and returns as perennial Bliss. The wearing of Udhi on our forehead helps to have dispassion in wordily life. Udhi is the blessing given by Saibaba both on spiritual and material plane in one’s existence. It is also a cure for both serious and simple ailments. Even death has been overcome by taking the Udhi, from Sri Shirdi Sai Baba.

Till today the fire at Shirdi is kept on and devotees take this Udhi for spiritual as well as material well-being. Udhi mitigates their suffering and sorrow and take heart for living in peace amongst all troubles and tribulations. This Holy ash or Udhi is most sanctified blessings of Baba.
Baba’s Vibootim Paramam Pavithram, Baba’s Vibootim Paramam Vichitram.
Let us all pray and surrender at His feet with pure divine thoughts. Leave your ego at the feet of Baba. He will do the rest for you.