The following page of the FF7 Remake game guide describes the accessories and bonuses they offer.
You can find the accessories in the box, buy them in the store and at Vending Machines. The accessories usually offer powerful passive character boosts, so it's worth using them on an ongoing basis, especially during more difficult battles. For example:
- The Bulletproof Vest absorbs turret damage, so it's perfect for fighting when you know there will be a lot of them shooting at you.
- Revival Earings resurrect a companion when he falls. The item will be destroyed - perfect for very difficult duels.
Some more powerful accessories are hidden, for example - Champion Belt increases maximum HP by 10% and strength by 5%. To get the item, you'll have to max out the minigames in Final Fantasy 7 Remake.