Simulacrum allows one to use the reincarnate spell to give it a soul.
I understand the usefulness of using limited wish to give it some knowledge of the original, but what good is a soul?
I looked into some books I have, but couldn't find an explanation.
Creatures without a soul can't be raised or resurrected. They also can't do stuff that requires a soul (what, exactly, requires a soul depends on your DM). Lastly, and most importantly, only creatures with souls are living creatures, and thus subject to a wide variety of beneficial and harmful effects, like Chill Touch, Charm Monster, Avoidance, and Regenerate while immune to a handful of others, like (notably) Transmute Water to Dust.
I would postulate that it may not be specified by the rules. As you have not given the system you are using (Assumedly D&D but the rules differ greatly between the editions). From a roleplay perspective, in a world where it is generally accepted and even provable that everyone has a soul and there is an afterlife, if you are trying to reproduce or create a person, most characters would naturally want them to have a soul as it would be cruel to deny a sapient being their shot at the afterlife.
From a mechanical standpoint it is entirely possible that many, if not all clerical abilities may require some kind soul to function and certainly to cast. For example, a character with no soul would not be affected by raise dead or Resurrection as both of those bring the characters soul back from the afterlife.
In addition, at least in 3.5, a simulacrum is described as only partially real. The reincarnate would likely make it fully real.
