[lachlan-sneff]

I read Permutation City a year or two ago, so bear with me.

I believe they run the computation on physical computers so that the people running in the simulation can, at any point, check if their reality is a descendant of the initial garden-of-eden configuration.

[cmehdy]

I think I can see it: they write the software without having a clear proof (that, say, it is mathematically inconsistent and will crash). Then they execute it for long enough for its self-test phase(s) to be reasonably validated (with all the resources they can get in the "normal" world), therefore practically reassuring themselves that there's a mathematically valid outcome (in a "formal verification" sense), which ensures at least one configuration existing with their wishes (the Garden-of-Eden one).

It does have the side effect of ensuring the reality that exists also has a way to know the Garden of Eden configuration (provided the people scanned are/were made aware of it prior to the scan, since it's made clear that there's no guaranteed way to know what happened to the "flesh person" after a scan when you're "reborn virtual".