Why do ready-to-eat, store bought hams have to be filled with water?!
Answers: It makes for a more tender, and as you stated, ready-to-eat ham. As an alternative, you can purchase Virginia sugar cured hams, soak in water several times and bake. It's a bit salty and you have to prepare it, but you get more ham and less water.
water is cheaper than meat. Added weight = $
Because it turns out to be better product. Yea I know, your paying for water weight, but it's a more user friendly product
Because if 'they' didn't add the water the purchaser would only be paying for what he wants, namely, ham. Isn't life in the business lane fun!
Notice, too that those blimped-up potato chip bags could be full and still be protectively (against crushing) inflated.
The official answer is to make the meat tender and less dry, while the actual answer is to sell you water at the price of ham.