The addition of two identical signals with a delay between causes constructive and destructive interference between the two signals. These interferences cause strong notches at the destructive frequency points.
But in reality these effects are much less apparent. Only if delayed signals are mixed electronically, the effect is strong as illustrated above! Room acoustics with a full set of reflections (delayed signals) are blending into any real world situation creating only small notches that are hard to detect in a measurement.
Also the effects of psychoacoustic should be mentioned: reflections within a certain time frame are not received as comb filters but as supporting elevation of the original signal!
typical comb filter during recording
audible comb filter on reflective side wall