Question

generally auditoriums are provided with materials that produce reverberation.Why​

Answer 1

when there are hard walls inside the room, it leads to arise of lots of undesirable acoustic effects.

Now suppose there is one hard wall inside the room and all other surfaces are soft, when the sound emits you can easily recognize standing waves with area where the sound is high and where its low. Generally, in auditoriums material is used that reverberate sound waves. These waves can help the listen to understand what speaker is speaking.