You make a little baby universe by pinching off part of the normal universe.
Mini-Big-Bang makes the universe expand, along with the nanites you shoved in there. The nanites consume the rich hot plasma to make high-speed computing circuitry and then attempts to solve some problem. Eventually, the baby universe begins to contract because it can't expand indefinitely, and the computer is forced to become more and more durable while still solving the problem.
Due to the way it works, it allows you to shove a near-infinite amount of thinking in a finite amount of time.
Great at solving problems.
A really big Tipler Oracle could last a very, very long time. If it was given the means to communicate with the outside world through the wormhole that leads to it, it could be a NPC of sorts.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather