Mana Flower

The mana reduction is the small half of it. The reason to wear one is the automation: the instant a spell costs more than you have left, it drinks a mana potion for you and the cast still goes off. You never stop firing to open your inventory.

Every one of those potions inflicts Mana Sickness, and stacked up it can strip half your magic damage for as long as ten seconds. On a weapon you hold down continuously, the flower can quietly suppress your damage while it keeps you casting. Celestial Cuffs, which refill you from damage taken instead, avoid that entirely and are the better pick in most fights.

Weapons with a long spin-up and a brutal drain, the Last Prism and the Laser Machinegun above all, are the case it was made for. Dropping the beam to chug a potion costs you more than the sickness does. Outside those, the Quick Mana hotkey does the same job for free and leaves the slot open.

If you are clearing out spare Nature's Gifts, crafting them up into flowers first is worth more than selling the parts separately.