IV bags are there to get you out of the shit. In the UK its pretty rare to get an IV bag, unless you are a celebrity.
EMT here. Venous access is mandatory protocol in many emergent cases, such as trauma and cardiac. If they already have hypovolemia, eg bleeding, it's not going to get better by itself. Reduced BP, impaired perfusion, or anemia will all end up shorting the brain and heart muscle eventually, so they want to stay ahead of that. Also, if a surgery is expected later, they want an empty stomach because vomiting is not good when inhaled. Another reason is for drug delivery, even if it's just an "saline lock" (no bag, just a port for later) to insert meds. I think oral fluids are great, but only when indicated.