Positive planes of existence and reincarnation on Earth
From the planes of existence below Mahalokam, people need to be reincarnated on the Earth plane to settle their destiny and complete their give-and-take accounts.
If one attains Mahalokam and Janalokam after death, that means they are above the spiritual level of 80%. These souls do not need to be reincarnated as all their remaining destiny (accumulated account) can be worked out from these planes itself.
However these evolved subtle bodies may choose to be born of their own will. They do so primarily to act as spiritual guides for humanity.
The importance of the Earth plane of existence
The Earth plane of existence is very important. It is the only plane of existence where we can make rapid spiritual growth and settle our give-and-take account in the shortest period of time.
The main reason for this is that with the help of the physical body, we can do many things to enhance our spiritual growth and spiritual level and reduce the basic subtle tama component.
Other than Earth, spiritual growth is mostly likely to occur only in the regions beyond Heaven such as Mahaalokam etc.
This is because in Heaven, the subtle bodies run the risk of getting caught up in the unending pleasures it offers. In the Nether and Hell planes of existence, the punishment is so severe and also the distress from the other higher-level ghosts is such that it becomes very difficult to rise above the suffering to undertake any spiritual practice of value.