• Scientific specimens, to better understand what alien intelligent life might be like; dissecting and scanning the brains and bodies of existing humans seems like it would preserve more information, but further scientific studies could involve the creation of new humans or copies of old ones
• Trading goods, in case of future encounters with alien intelligences that are interested in or care about humans; one could try to fake this by dissecting the humans and then only restoring them if aliens are encountered, but perfect lies might be suspicious (e.g. if one keeps detailed records it's hard to construct a seamless lie, and destroying records is suspicious)
• Concern that they are in a stage-managed virtual environment (they are in a computer simulation, or alien probes exist in the solar system and have been concealing themselves as well as observing), and that preserving the humans brings at least a tiny probability of being rewarded enough to make it worthwhile; Vinge talks about the 'meta-golden-rule,' and Moravec and Bostrom touch on related points

Storing much of humanity (or at least detailed scans and blueprints) seems cheap relative to the resources of the Solar System, but it could be in conflict with things like eliminating threats from humans as quickly as possible, or avoiding other modest pressures in the opposite direction (e.g. concerns about the motives of alien trading partners or stage-managers could also favor eliminating humanity, depending on the estimated distribution of alien motives).

I would expect human DNA, history, and brain-scans to be stored, but would be less confident about experiments with living humans or conscious simulations thereof. The quality-of-life for experimental subjects could be OK, or not so OK, but I would definitely expect the resources available to live long lifespans, sustain relatively large populations, or produce lots of welfare would be far scarcer than in a scenario of human control.

The Butler citation is silly and shouldn't be bothered with. There are far more recent claims that the human brain can do hypercomputation, perhaps due to an immaterial mind or mystery physics that would be hard to duplicate outside of humans for a while, or even forever. Penrose is more recent. Selmer Bringsjord has recently argued that humans can do hypercomputation, so AI will fail (as well that P=NP, he has a whole cluster of out-of-the-computationalist-mainstream ideas). And there are many others arguing for mystical computational powers in human brains.