Please cite texts from where you write something as well if possible.
It is said in the Manu-smriti 2.108:
"Let an Aryan who has been initiated, (daily) offer fuel in the sacred fire, beg food, sleep on the ground and do what is beneficial to this teacher"
Here is an example from the Manu-smriti 6.26 which says that an ascetic should sleep on the ground:
"Making no effort (to procure) things that give pleasure, chaste, sleeping on the bare ground, not caring for any shelter, dwelling at the roots of trees."
See also Manu-smriti 6.23 and 24:
"In summer let him expose himself to the heat of five fires, during the rainy season live under the open sky, and in winter be dressed in wet clothes, (thus) gradually increasing (the rigour of) his austerities."
"When he bathes at the three Savanas (sunrise, midday, and sunset), let him offer libations of water to the manes and the gods, and practising harsher and harsher austerities, let him dry up his bodily frame."
Why are all those austerities required? This is answered in the verses Manu-smriti 6.74-75:
"He who possesses the true insight (into the nature of the world), is not fettered by his deeds; but he who is destitute of that insight, is drawn into the circle of births and deaths."
"By not injuring any creatures, by detaching the senses (from objects of enjoyment), by the rites prescribed in the Veda, and by rigorously practising austerities, (men) gain that state (even) in this (world)."
So the answer would be that the austerities such as sleeping on the bare ground, etc, are needed for a person in order to become free from the consequences of his own deeds, namely karma, and in order to achieve liberation from this world, ie that is the liberation from the circle of births and deaths. So to speak, those austerities are a religious practice that leads to the final liberation (mukti or moksha).
There is yet another reason why austerities should be practiced as a religious practice in one's life. It serves as a kind of a reminder that life in this world is not a life in a place intended for a comfort and enjoyment, but this world is a place of discomfort and suffering. One who sleeps on the bare ground, etc, will feel how that is uncomfortable, and thus he will be constantly reminded that in general the nature of life in this world is discomfort, pain and misery. And this will lead him to something that is called vairāgya "renunciation", ie the renunciation from sensuality, or you can call it renunciation from this world. The idea is that one realizes the nature of life in this world as discomfort and suffering, which will help him to abandon any desire for the sensuality and enjoyment in this world. This is a very important concept because precisely because of desire for the sensuality and enjoyment in a variety of objects for sense gratification in this world people continue their lives in this world in the circle of births and deaths, which is the world of material miseries. How much important this is can be seen from the above mentioned verse from the Manu-smriti 6.75 " ... by detaching the senses (from objects of enjoyment), ... , (men) gain that state (even) in this (world)."
- See quotes from the Manu-smriti at http://www.sacred-texts.com/hin/manu.htm