The middle option only exists so you buy the big one.
With the decoy effect, a seller slips in a deliberately unattractive option. Small coffee €3, large €4.50: many pick the small one. Add a medium at €4.20 and the large suddenly looks like a gift, and choices shift upward. The decoy is rarely bought; its only job is to move the comparison.
Useful: When an option is obviously pointless, ask what it's really there for.
Source: en.wikipedia.org