Two distinct uses: directional bearish speculation and portfolio insurance. The speculative version is symmetric to a long call — buy 0.50–0.70 delta puts at 30–60 DTE on a stock you have a bearish thesis on, exit at 50–100% profit or 50% loss. The insurance version (the protective put) is bought against a stock you already own, hedging downside while leaving upside intact. The two use cases share mechanics but have different position-sizing logic.
Be aware that puts trade with a volatility skew — OTM puts on equity indexes are typically more expensive than the symmetric OTM call because institutional hedgers bid up downside protection. That skew is real edge for sellers and a real cost for buyers. Account for it before you place the trade, not after.