Why does a worse option you would never pick still change your choice?
Sellers add a bad third option not for you to buy it, but to push you toward the one they want.
The decoy effect happens when a third, clearly inferior option is added to two choices. The decoy is worse than one option in every way but not directly comparable to the other, which makes the option that beats it look like the obvious deal - even though that option did not change at all.
A coffee shop lists small for 35k and large for 65k. You hesitate. Then a medium appears at 60k - barely cheaper than large but much smaller. Suddenly the large feels like a steal, and you buy it.
A worthless option you would never choose can still steer you, because it changes what the other options look like by comparison.
Menus, pricing pages, and subscription tiers often plant a decoy to nudge you to the pricier pick - spotting it lets you compare the real options on their own.
Decoy = a loser planted on purpose to make one winner look obvious.
Learn the idea and practice English at the same time.