Since DMRS is also being used as a reference signal in PUCCH and PUSCH channel, so why an extra SRS signalling is required.
1 Answer
Because DMRS is only sent if there is data to transmit and, therefore, narrowband while SRS, as the term sounding suggests, is not associated to any data transmission and also wideband.
For example, SRS can be used by base stations to choose the best portion of the UE channel bandwidth to schedule next PUSCH.