The short call pays for part of the long call, which lowers the breakeven-price compared with buying the call outright and removes most of the vega exposure. What you give up is everything above the short strike.
It is the structure to use when you have a price target rather than a hope. Setting the short strike at the target converts an open-ended bet into a defined one, and the max-profit is simply the spread-width minus the net-debit.
Example: XYZ at $50. Buy the 45-day $50 call at $2.30, sell the $55 call at $0.80, for a $1.50 debit. Max loss $150 per spread, max profit $350 at or above $55, breakeven $51.50. The naked $50 call would need $52.30 just to break even.
Related: debit-spread, bear-call-spread, vertical-spread, max-profit