In this paper, we emphasize why it is important for future neutrinoless double-beta ($0 ubetabeta$) decay experiments to reach the sensitivity to the effective neutrino mass $|m^{}_{betabeta}| approx 1~{rm meV}$. Assuming such a sensitivity and the precisions on neutrino oscillation parameters after the JUNO experiment, we fully explore the constrained regions of the lightest neutrino mass $m^{}_1$ and two Majorana-type CP-violating phases ${rho, sigma}$. The implications for the neutrino mass spectrum, the effective neutrino mass $m^{}_beta$ in beta decays and the sum of three neutrino masses $Sigma equiv m^{}_1 + m^{}_2 + m^{}_3$ relevant for cosmological observations are also discussed.
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