You want the stock to gap down after you write the put as it's your intention to buy the stock on a dip. Some gaps wont bring the price all the way down to the strike price but you hope that it continues down and you get your stock at expiration. Some gaps will go past the strike price but the stock moves back up before expiration; you don't get your stock, but at least you made some money. The only way you can do worse than simply buying on the dip is when the stock gaps down from above to below the strike price minus the premium and stays down until the stock is put to you; had you waited for the dip without using a put, then your limit order would have bought you the stock for less by an amount equal to the strike price minus the premium minus the post gap price. This would be a rare occuance and extremely rare for it to be a large amount; over time, its effect would be trivial in comparison to the option income.
The main trade off is the lost opportunity when the stock dips below the put's strike price but then rises and the put expires worthless; but you get your premium and while you lose the opportunity to own the stock at the dip price, at least you made money without the risk of owning the stock after expiration day. This strategy is not only more profitable than just buying the stock, it's somewhat more conservative too.
This is a fine strategy for a stock that has low volatility; a lower put income is in line with lower profit goals of buying these kind of stocks; and buying volatile stocks by selling puts, while risky, is still overall a bit less risky than buying them directly.
Those all sound good in paper, but let assume the following case in reality;
Current stock price $40
The stock provide a 5% dividend with low volatility and good P/E ratio and etc, e.g. A good candidate to hold in long term
You write a put for $38 strike for €$0.50
The company annouce a bad earning and decide to stop the dividend payment due to "accounting and bad management issue“. The CEO get sacked.
The stock gap down to $36 post earning and move lower "gradually ' (without pullback) to $33 in expiration
Shoud you will follow your strategy, you will be assigned the stock at $38, with the net loss of $4.50 (38-33-0.5).
The stock seems will go down further based on the analysis recommendations with price target of $20
Question : Are you going to hold the Put until expiration post earning or rather taking a loss by buying back the Put before expiration?
No one will write a put for a stock that move lower (similiar as no one will write a Call in SPY in bull market). The problem in put writing strategy is you collect small premium, but when sh** hit the fan, you get caught and force to own a bad stock which WAS supposed a good stock before.
You can argue this is no different as buying a "good" stock that become a "bad" stock, but this is completely different story ( i never buy and hold any "good" stock as i have different investment strategy, I am not Warren Buffet

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