A call spread is a bounded bullish bet: cheaper than the outright call, with profit capped at the strike difference.
Why use one. The long and short vega largely offset, so a spread is far less sensitive to implied volatility than an outright option. If you have a directional view but no volatility view, a spread expresses it more purely - and it is much less exposed to a vol crush.
The technique for any structure: draw the payoff diagram. Maximum loss, breakeven and expiry outcomes all become immediate, and are error-prone without it.