When superannuation death benefit payments are made to non-tax dependants (i.e. most adult children), tax can be payable on certain components. In the calculation of this tax, it includes the Medicare levy (currently 2%).
Without getting too technical, if this same benefit is left via a Will, the Medicare levy is not payable. Thus creating a tax saving of 2%.
There are also Centrelink reasons why certain beneficiaries might benefit from receiving a superannuation death benefit via a properly constructed Will, rather than directly from a superannuation fund.
Of course this isn’t a one size fits all issue and as usual we recommend you seek the appropriate professional advice.