Why is water so widely used as a solvent?
Because most materials display some solubility in the stuff.
Water is distinguished by (i) its physical size; and (ii) by its involatility, which is absurdly high for such a small molecule. Compare the boiling point of water with those of other small molecules,
Most materials, polar or not, express SOME water solubility. For experimental chemists, most of the time, the challenge is to dry their reaction solvents, and remove the water.