The extra short contract is what pays for the structure, often turning a debit into a credit. It is also what makes the position dangerous: past the short strike the surplus short option is effectively a naked-call or naked-put, with undefined-risk on that side.
Ratio spreads suit a view that the underlying will move some but not far. Their payoff-diagram rises to a peak at the short strike and then falls away, crossing back into loss if the move keeps going. Broker approval for the naked leg is required, and buying-power-reduction reflects the unhedged contract.
Example: XYZ at $50. Buy one $50 call at $2.30, sell two $55 calls at $0.80 each, for a $0.70 debit. Best case is $430 with XYZ at exactly $55. Above $59.30 the position loses, and it keeps losing dollar for dollar all the way up.
Related: backspread, ratio-write, undefined-risk, broken-wing-butterfly