In Doctor Strange, when Mordo and Wong are explaining to Strange about Dormammu and the Dark Dimension, they say he wants to devour all the worlds in all dimensions, and there is no world he wants more than Earth.
They never say why, though.
Do we have any information, preferably from the writers and producers, or if not, from the comics, explaining what makes Earth specifically so desirable a target? Wouldn't Asgard be yummier, for example?
It's never said why, but it is implied
We find through the movie that
The running implication as a result is the possibility that Dormammu hungers for Earth because there is an "immortal" being drawing power from his dimension on Earth. In a way, the Ancient One makes a choice based around being more powerful to defend Earth, and the consequence of her choice is to put the Earth in more danger - and also prolonging her life in a circular way leads to the events that result in her death.
Which is a pretty fitting theme all around for a movie involving time, time loops etc.
It is possible that this line was more meant in the context of Dormammu seeing earth as a threat to its own power, and thus with Earth eliminated, Dormammu could further flex his power unopposed.
The existence of the Kamar-Taj (an organization specifically created to fight Dormammu and his influence), Infinity stones (there are two on earth at the moment, including the Time Gem
which proved to be his undoing
) are all likely reasons why Dormammu would see Earth as a major threat, and thus his main target.
