Why do acids and bases form ions when they dissociate in water

If the acid donates a H^+ to H2O forming a hydronium ion, that carries a + charge (and leaves a negative remnant); e.g. HCl + H2O ==> H3O^+ + Cl^-

If the base accepts a H^+ from water, that forms a positive ion with the base and leaves the OH^- as the negative remnant; e.g., NH3 + HOH ==> NH4^+ + OH^-