The longer a capital gain is deferred, the longer the pre-CGT investment can compound.  As soon as you realise a gain you become liable for a capital gains tax liability, which is money that must be paid to the tax office instead of sitting in your account growing and earning you dividends.

Deferring capital gains produces a time value of money type benefit by deferring a liability, it also provides for some people the opportunity to realise the gain in a lower tax year, such as after they retire.

It makes particular sense to defer sales in super funds where there is no CGT realised on rolling over from accumulation mode to a pension.  Once a super fund goes into pension mode it becomes tax exempt, meaning that capital gains realised after the fund goes into pension mode will be CGT exempt, even though the capital gains occurred over many years while the super fund was in accumulation mode.