When Tom, known as Lord Voldemort, gained access to the Black family library, thanks to Walburga's generosity, he was closest to being happy. Knowledge was power, and he craved power the most.
During his search through the darkest magic and rituals, he found a ritual that can be used only by Blacks. At first, he discarded it. No matter how deep into a dark magic he was ready to dive, fucking up with the natural order of time was never a good idea.
Years later, after his first death and more than thirteen years of surviving with no body, thanks to the soul magic and horcruxes, he found himself almost ready to mess with it.
Multiple encounters with a Potter boy cut deep into his self-preservation, and he found himself thinking about it more and more.
When he found out that some of his horcruxes were missing and Potter-boy stole another one from Gringotts, he was moving forward with it.
Of course, he did not use it as is. He got an original idea, which focused on Black family magic, modified ritual to anchor it around his own body and set a time anchor to the first meeting with Bellatrix Black.
So he was pretty sure that in the unlikely case of his final death, charged ritual would burn Bellatrix's body, soul and magic to throw her memories back in time, where he would have only to read them and adjust his plans accordingly.
He was sure this failsafe would not be needed, but he was Slytherin through and through, and Slytherins always have backup plans for backup plans, if they are cunning.