Ganesha is known as Vighneshwara because the devotees believe that Ganesha roots out obstacles and difficulties. There are many stories, which narrate how he saved even the gods.

Ravana was the king of Lanka. He was very strong. Once there were indications that the gods would suffer at the hands of Ravana. 

Ravana was a devotee of Ishwara. Once he performed strict tapas. Pleased with his 
tapas Ishwara appeared before him and said, "What boon do you desire?" Ravana begged for Shiva’s Atmalinga (Shiva’s Atmalinga was the reflection of Shiva in the form of the Linga. The presence of this Linga is as good as that of Shiva Himself). It was Ravana’s conviction that nobody could defeat him, once he acquired the very power of Shiva.

Shiva had said, "Ask for whatever boon you choose." Therefore he was bound by his words and had to give away his Atmalinga. He, however, cautioned Ravana: "Don’t put this Atmalinga on the ground even for a moment; once you put it on the ground you cannot get it back." He then disappeared.

The gods trembled to see Shiva’s Atmalinga in the hands of Ravana, who was a menace to the world. Ravana was their bitter enemy; and now, with the Shivalinga itself in his possession, they were in great danger. They were troubled not knowing who could protect them.

Then they remembered Ganesha. They went straight to him and explained to him their troubles. The kind Ganesha consoled them. He put courage into them saying, "I shall not allow Ravana to get Shiva’s Atmalinga Have no fears." The gods went away and Ganesha followed Ravana to carry out his mission.