You can use it in sens fortress to get past the demons. It's under the first set of swinging axes on the bridge and there's a lair of tar slowing u down.

It is very useful in the Hydra fight

much like this comment

Not really necessary at all, get up close till it stops spitting water at you and just block until the heads attack them swing and repeat. Don’t need to move at all

Undead Asylum F2 West key. It's on top of one of the buildings in firelink shrine and you can get to it from the same staircase by the the nest that takes you back to the Undead Asylum again.

The key is on the roof of the building at firelink shrine. The building that the Way of the White dude is in. Its accessible once you send the elevator down from Undead Parish.

You can easily go back to the asylum with some parkour and the key is on the bodu on top of that petril dudes head

It's also useful at the bottom of Sen's fortress when trying to fight the titanite Demons

Damn Nazis