macd has a 9 period signal line, the stochastic-oscillator has %D, and most oscillators offer one. The purpose is to convert a continuously varying number into discrete events.
A signal line always adds lag, because it is an average of something that was already an average. A crossover therefore confirms that the indicator has already turned, and by the time it prints, price has usually moved.
The value is consistency rather than timing. A crossover rule is unambiguous and testable, which is worth something compared with judging by eye whether a line looks like it is rolling over. Just do not mistake the extra line for extra information.
Related: macd-signal-line, stochastic-oscillator, indicator-lag, macd, confirmation-signal