It was originally advice to avoid complications for female cats, ignored it but later noticed energy loss when feeding fish for several days to a week. If you use store wet food (my cat would not eat land meat from frozen/raw, has a taste for turkey, and got tired of fish herself) they are already supplemented with oils and omega-3 and where I am there was not a single brand without them and other garbage.
I just find it hard to feed pets the usual pet foods - the pelletized ones = to either my cats or my koi. Lucky for me, where I'm at, I can always go to the public market and buy fresh fish 10kg at a time. The frontal part for the koi, the hind part for the cats. Koi eat the fish fresh, cats eat them cooked. The fish is always saltwater fish, as it guarantees they're wild and not fed lousy pellets at fish farms. They also eat coconut meat, for koi it's diced, for cats it's grated at the public market. The koi are good with it, no need for supplements, although I'll add electrolytes to the pond water for minerals. The cats have supplements I mix myself - salt, taurine, calcium carbonate, vitamin b1, b2, and b3
But if I were to move and not have access to saltwater fish in the public market, it would be hard to care for my cats and my koi. As it is, it's hard to care for a dog, unless it's a small dog, but I like larger dogs, and I'd have to spend a lot to feed them. Perhaps if I have a farm, I can have all of them. I can't imagine any pet eating pellets. If I don't eat pellets, there's no reason they should.