MMP means that New Zealand’s most popular PM ever has an one seat majority. So if National lose an electorate MP and then lose a by-election there could be a fresh election that would be very hard for National to win.
This means they can’t push Nick Smith too hard, as Nick 4 Nelson holds a seat that if Labour and the Greens were to collude on Labour could easily win a by-election. MMP doesn’t replace an electorate seat lost in a by election with a scum List MP.
It is not just Nelson that could cause a problem. If an MP dies, or commits a crime or just decides they are sick of being passed over for promotion they could leave, force a by-election and then an election. Those in safe blue seats don’t really have much power because National would win a by-election.
Those in marginal seats like Nick Smith do have a lot of power, as if they leave they could bring down the government.