Flare of Fortitude protects your board and life total during a critical turn. It gives your permanents hexproof and indestructible until end of turn while preventing your life total from changing. You can pay four mana or sacrifice a nontoken white creature instead, making it useful when you have committed your mana to developing the battlefield but still need a defensive response.
White creature and token decks often have a small utility creature available to pay the alternative cost. A creature that has already supplied an entry ability can be a reasonable sacrifice if it saves a much larger board. Tokens do not qualify for that payment, so a deck consisting mostly of white tokens still needs actual white creature cards if it wants to cast the Flare without mana.
The combination of hexproof and indestructible handles many targeted removal spells and destroy-based sweepers. It does not stop every form of interaction: an untargeted exile effect, a sacrifice instruction, or a toughness reduction can still matter. Choose the response based on what the opposing spell actually does rather than assuming any protection spell answers every board wipe.
The fixed life total can also preserve you through a large damage event, but it prevents life gain as well and can stop you from paying life costs. That matters in decks using life as a resource. Flare is strongest when the protected permanents are valuable enough to justify the card or creature spent defending them.
