Yes, travel insurance matters for a resort stay in the Mana Island area of Fiji's Mamanuca group, and the honest reason has more to do with geography than any particular property: this is a small offshore island reached only by boat from Nadi on the main island of Viti Levu, and that access detail shapes the whole risk picture more than the resort experience itself.
The Mamanucas have only basic medical facilities of their own, and anything beyond routine care typically means a boat transfer back to Nadi, the region's real hospital hub on Viti Levu, with further evacuation for anything serious usually meaning Auckland or a hospital in Australia. Confirm your policy explicitly covers inter-island boat transfer and international evacuation, since a routine policy that only reimburses local clinic visits misses the part that matters most in a setting this dependent on boats rather than roads.
Snorkeling and diving on the surrounding reefs are the main draws for most visitors to this area, and both carry the usual water-activity risk that some general policies cap or exclude at depth. Confirm your policy names diving specifically and check any depth limit before booking a reef excursion through the resort, since a casual introductory dive still counts as the kind of activity insurers scrutinize, and resort-arranged trips are not automatically covered just because they were booked on-site.
Fiji's cyclone season runs roughly November through April, and storms in this window can disrupt boat transfers between Nadi and the outer islands, sometimes stranding visitors on one side or the other for a day or more with limited onward options. Trip-delay cover is worth checking closely if your visit falls in this window, alongside cancellation cover for any pre-booked resort package. Keep any resort or clinic paperwork, boat-operator delay notices, and receipts for unplanned accommodation if a claim follows.