Before starting the day, or keeping feet on Earth, Hindus usually touch Earth by their hands. What could be the possible reasons behind this ritual?
Is this to show respect to Earth?
Purusha Suktham and Sri Vishnu Sahasra namas say the earth His feet. That is why we bow to the earth, as it amounts to touching the Lord. Another reason is Bhoomi or Bhoomadevi is the consort Sri Mahavishnu - Both the Bhagavatha Purana and Sshasranama tells about this. Mahavishnu is called Madhini pathi (husband of Bhooma devi). It is like touching the feet of one's own mother. For mother gets the top priority in the order of reverence (mathru devo bhava, next comes father Pirthu devo bhava, next comes the teacher Acharya devo Bhava and lastly the God. Mother is the own who gave you the biological form, father leads by example, teacher shows the world and lastly God enables you to merge with Him.
samudravasane devi parvatasthana mandale vishnupatni namastubhym padasparsam kshamasva me
O Goddess, who dwells in the Oceans, who's life-giving breasts are the mountains. Salutations to the wife of Lord Vishnu, Forgive me for the sin of touching you with my feet.
Among traditional South Indian Brahmins, it is customary to recite this verse before rising from bed. You touch the earth and seek forgiveness from Bhu Devi for stepping on her.
Not all hindus touch the Earth before starting thier day but few do that. This is related to the belief that Earth is Prithvi (Hindu Goddess). And touching the Earth means touching feet of goddess. Earth is also considered as mother Earth in Hinduism. This touching at the start of the day symbolizes thanking Earth for giving us everything for survival.
For the same reason we worship God every morning, and for the same reason people do Surya Namaskar etc.
In Hinduism, the mother Earth is considered a goddess. This is just a kind of worship, similar to how you worship any other God/Goddess.
As for why we do that first, with hindsight, we can argue that mother Earth is what we encounter the first at the start of the day. So we do that!
It is because Hinduism teaches us to respect every thing. This planet earth is also like our mother. She gives us food, she gives us shelter, she gives us different varieties of things for our existence. So karmically it is important to express gratitude to her. That is the reson behind that ritual.