Why do we use disodium EDTA?
Because generally, we don't need all six teeth to bind to a given target in practice, and because EDTA by itself is poorly soluble in water. Disodium EDTA is water-soluble.
EDTA, or ethylenediamminetetraaceticacid, a hexadentate, chelating ligand, looks like this:
So you can see that four "teeth" on EDTA belong to the acetate oxygens, and two of them belong to tertiary amine nitrogens. EDTA has a high complexation formation constant, which means the equilibrium is highly in favor of EDTA binding.
However, EDTA by itself is NOT very water-soluble.
In disodium EDTA, or
In practice, it is generally more useful and convenient to have EDTA partially ionized as disodium EDTA since it is easier to dissolve in water for a complexometric experiment.