Lord Ganesha is believed to be the swami of all the 'ganas'. According to astrology, he is considered to be the swami of Ketu and all the resources on this earth.
According to Hindu scriptures, Lord Ganesha is worshipped before beginning any work or ritual. It is believed that worshiping Ganesha before starting any work ensures that the work gets successfully finished.
He is the only God who is offered 'durva' as it is believed that offering durva appeases Lord Ganesha. Lord Ganesha is offered 21 stacks of durva, each stack consists of 21 durvas.
Very few people know that there is a story associated with this ritual. Once upon a time, there lived a demon known as Analasur (anal means fire) who used to trouble the sages and saints. Fed up of this demon, all the ‘devas’ request Lord Ganesh for help.
In order to protect the world, Lord Ganesha gobbled the demon. But, as soon as he did so, he felt the burning sensation inside him. The sensation was so strong that he couldn't bear it at all.
Kashyap muni gave him 21 stacks of duvra to eat and Lord Ganesha was relieved off the suffering. Since then it is believed that offering 21 stacks of durva appeases Lord Ganesh.
Durva has medicinal qualities that help cure many stomach problems and even mental ailments. It works as an antibiotic. Even seeing and touching durva gives mental peace.