It is part of Scientology philosophy that one is basically an immortal, immaterial spirit, a thetan, inhabiting a meat body for the allotted three-score years and ten, more or less, and then when that body has ceased to function one gets another one.
There are various explanations for the general occlusion of memories of previous lifetimes. These occlusions are often resolved in auditing, and most people journeying to Clear view past lives along the way.
The first good view of a previous lifetime, where one has some good degree of certainty that this is what it is, is often a big moment for an individual: He now has some personal reality on the idea that he is, in fact, immortal, and will continue to exist in a self-aware form after the death of his current body. This is usually a considerable relief.
Being told by another that one has lived before, maybe with details, is evaluation and may well not be true. Seeing it for oneself is not the same.
These past lives may be exciting or humdrum, the characters involved important or everyday.