Why did the WAU create/revive Simon?

For what purpose did the WAU took an old legacy sample and put it into a random dead body mixed with structure gel, causing Simon's revival?

Is it yet another attempt to create a proxy or an attempt to create new life on the station, as a way of preserving life? Indeed Simon has no relation to the Pathos-II.

Or was Simon revived by someone/something else?

I think there's enough evidence to support the idea that WAU was trying to keep the human race alive. It was created to run the installation—including maintaining life support and all that goes with it—and it reacted to the destruction of life on the surface by doing whatever it could to prolong human life below.

Unfortunately, it had very little to work with, and some of the humans were actively taking their own lives. I couldn't tell if WAU had killed some of the humans in order to use their bodies. That they were placed randomly around the station (no doubt a design choice by the developers) made it look like the structure gel had burst through and grabbed people, but the dialog made that seem unlikely.

In the case of Simon (and probably the other three people's scans in the system with him), it looks like WAU simply took advantage of the extra scans to keep more people alive. One major piece of evidence is that at some point I was attacked by a monster, but instead of killing me it stuck me in some structure gel just like the other people you see lying around. If the WAU had been sentient, it might have expressed concern that Simon was running around the station endangering his life at every turn. Immobilizing Simon was clearly WAU's response to keep him safe.