Sheetrock vs. plywood?

The room is not air conditioned and there is a lot of natural humidity.  Sheetrock will crumble from absorbed moisture.  The exterior grade plywood not only resists humidity but is structurally stronger than sheetrock and can take a beating, like you might have in an enclosed porch

That may be ok for an outside added on porch or patio area.  But if there is any insulation in those walls' plywood my not jive with the local fire code. Sheet rock is best for fire protection inside the home, but doesn't stand up well to a porch foundation and weather. I'll never forget the first time I looked at the building code at city hall. Its was an incredibly large set of 108 hard bound books. It would take me 3 years, 8hours a day to read through it all. Plywood is much stronger than sheet rock, if you can keep it dry.

As others have said, it will withstand the moisture of an enclosed porch much better, it will be stronger and last longer. If you want to put up a shelf, picture or a flower basket or anything else to make your enclosed porch more enjoyable to you, the plywood, if it is 5/8 or thicker will be strong enough to to hold it and you won't have to worry about finding a stud. 

Many people, including myself prefer the look of wood over painted drywall. Find a natural stain for it, if its not stained already, and bring out the richness of the grain and it will really look nice.

Sheetrock, if it gets wet, is worthless.  It will barely hold itself up on the screws.  Get it wet a lot and it sags, crumbles and degrades to junk you have to remove and throw away.

Plywood, especially exterior rated plywood, suffers from none of these shortcomings.

https://homeguides.sfgate.com/drywall-vs-plywood-w...

So: there are pros and cons to plywood

the primary "pro" being its strength (for attaching shelves or whatnot)

the primary "con" being its flammability.

It depends on how weather tight the room is. It it is fully and properly enclosed so it is not subject to the elements then the contractor could have used drywall or panelling. If it is subject to the weather then the plywood should be an exterior grade.