Located deep within the Perseus Veil, the planet has a unique envoirnment perfectly suited for Quarian physiology. The planet came under the control of the Geth and has been for hundreds of years ever since the Geth uprising started with the Morning War when they rebelled against their Quarian creators and drove them into exile. This led to the creation of the Migrant Fleet, a collection of ships that house a majority of the Quarian race.
Rannoch is unique in that it lacks any form of insect life. In order for its plants to pollinate, the pollen must be transmitted physically by an animal species. Long before the Morning War Quarians physically pollinated plant life all over the planet. The task of keeping the planet's plants alive had a drastic effect on the Quarian population, who adapted to dealing with the pollen and spreading the pollen, affecting their immune system on other worlds.