I use a bit of it for fun, to make the game a little more challenging, but not too much (because the bossfight design sucks)

There's always a reason to use Vow of Frenzy (All foes are +20%/+40% faster), since it makes your runs faster. Literally, your experience room-to-room is quicker.

It's somewhat comparable to playing Vampire Survivors and increasing the game speed. If you're unlocking the true ending or epilogue, or trying to do a specific task/Prophecy, it just makes things generally faster.

There's also unique dialogue scenes/interactions with the Vow of Rivals bosses enabled, which are not accessible on normal mode/0 fear. Medea + Polyphemus have a bunch of unique dialogue, for instance.

There's also rewards available in Tartarus and the Summit while enough Fear is active (Gifts of the Night), like Witch's Delight.

Yes, you can create this stuff at the Cauldron at the Crossroads with the right Incantation, but technically it's a bonus to running Fear, which is what you asked.

The biggest reason is fun.

Mel scales to a point where 0 Fear/story mode isn't difficult anymore, and she just rolls over everything with full Arcana, maxed out weapons, familiars, etc. Same deal with Zag in Hades 1.

The game is way more dynamic and engaging with Fear/Heat enabled.