Why it works?
In lab, they put in something that binds with sodium to detect sodium. Why? 9g of NaCl is not 4.5g of Na and 4.5g of Cl. Why not? Remember the molecular weight of Na is 23g and Cl 35.5g. Therefore, to really know how much Na there is, it's hard to get it straight from blood. Why hard? blood contains many things like albumin, lipid, etc., Therefore, need to have a reagent that specifically binds to Na to really measure Na in blood. Therefore lab value for Na in blood is reported as mEq because 1 reagent:1Na molecule.
Note that 0.9%NaCl is 9g of NaCl in 1L of the entire solution. Also note that mEq in Normal Saline "contains 154 mEq/L sodium and 154 mEq/L chloride." (per http://www.rxlist.com/normal-saline-drug.htm). So that means NS is slightly more salty than Na level in blood. Then, why would you give NS? well, if you're not too dehydrated, then NS is fine but if you have kidney failure with dehydration, NS is not sooooo good for u because it's quite salty.
Wait, but you thought NS is isotonic solution? It is! It is isotonic because it's equivalent to the entire blood (remember blood has many other solutes other than just Na). So.... => Dextrose 5% 1/2NS is really the isotonic with less sodium levels than blood => that's why it's so popular.
~Wonder Me!
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