Rearranged put-call-parity: S + P = C + K. Owning shares and a protective put gives limited downside and unlimited upside — the shape of a long call.
It matters because it reframes the protective-put and the married-put. A trader who says "I own the stock and I bought insurance" holds economically the same position as one who simply bought a call, minus the dividend and plus a lot more capital tied up.
Example: XYZ at $50. Buy 100 shares for $5,000 and the $50 put for $2.10, total outlay $5,210. Or buy the $50 call for $2.30, outlay $230. Both cap the loss near $210–$230 and keep all upside above $50. The difference is $4,980 of capital and any dividend during the period.
Related: synthetic-put, protective-put, put-call-parity