Holding onto Siri data makes sense
When they improve Siri they can backtest it against old requests (at least if they saved it in enough detail) Having history of past requests would also improve Siri's responses in the future.
A 'personal assistant' without any memory of the past would be far less useful. Think about celebrities who have personal assistants, do you think it would work well if they had a different one each day? That's really our ultimate goal here is to someday (many many years from now) have Siri, Alexa etc. be able to act like a human personal assistant (other than physical running errands....though by the time this happens maybe they WILL be able to physically run errands)