What sometimes happens with criminal investigations is that someone suspected of an offence is interviewed and says nothing (perfectly legal) and then later when the case comes to court and they have had time to consider things, they come up with an explanation that they didn’t volunteer previously. (A cynic might suggest that the explanation might occasionally be suggested to them by other parties, but of course that may not be fair).
Prior to the current PACE caution it wasn’t normally possible for prosecutors to comment on the fact that someone had been accused of something and that they didn’t offer any explanation at that time, which an average jury or judge might have expected to them to do, and that only offering the explanation at trial might look a bit suspicious, to put it mildly. The new wording allows the prosecution to suggest that to a court.
So, it’s absolutely right that you don’t have to say anything when interviewed under caution. But you also need to consider what impact that may have on any decision to prosecute and on the view which might be taken by a jury or judge. Why didn’t the accused simply explain that at the time? Each case on its merits etc, but it can harm your defence, as the caution accurately says, if you don’t offer a perfectly obvious explanation at the time. That’s what many innocent people would do. So staying silent can increase your chances of being prosecuted. Something to bear in mind.