[Eurytus]

I don't think Sauron would need 2nds in command or people to step in for him and command his military forces. Nor would he need someone to 'bounce ideas off' so to speak.

However he would be in control of a massive kingdom. Since there is no evidence that Orcs are much cop as farmers it would make sense for Sauron's realm to have to import much of its food. It certainly did in the case of Mordor (from the East) and it is likely that much of the role given to the tribute realms beyond the Anduin would be to supply the troops and workers of Sauron's realm with food and materials.

Sauron is not going to want to be bothered with this type of micromanagement and it is here that I believe the power heirachy comes in useful. Quite simply Sauron's realm would be too big for him to worry about the 'small details' like the sourcing of food and supplies. That would be his lieutenant's job. Given Isengard's location, commanding the gap between Sauron's realm and the tribute realms it would be in prime position to control the flow of products, supplies and materials from the west to the lands controlled by Sauron.

That is the situation where I think the Mouth of Sauron would have come in.