There are lots of reasons, but the main one is that Superannuation alone is unlikely to offer most Kiwis the lifestyle they want in retirement.
Superannuation is currently $23,058.36 for singles and $34,916.96 for couples per year (before tax). To maintain the lifestyle you have now, you're probably going to need more than that.
If you're between the ages of 18 and 65 and enrolled in KiwiSaver, you are entitled to a 3% employer match (if you're employed) and a government contribution of up to $521.43 per year.
Average life expectancies are increasing, so chances are we'll need to have more saved up before we retire than previous generations.
Saving a little now can really add up! If you would like some personalised help, contact a financial adviser.