In Book V of the Odyssey, Ino saves Odysseus from drowning by giving him a magic veil.
Why? Ino is a water deity, I'd expect her to be aligned with Poseidon's wishes. What reason does she have to intervene in favor of the hero instead? Even Odysseus seems to be puzzled about this, and doesn't immediately accepts her help. He only uses the veil after Poseidon (ironically) makes it absolutely clear that there's no other option.
First, the expectation is misguided. Deities are not beholden to their "kings." This is no better represented than all the Olympians disobeying Zeus by sneaking into battle in the Iliad.
In real life, Ino-Leucothea was a goddess of the sea that the Greeks venerated, one of many that they tried to ask for good sailing. The sea was notoriously dangerous (see the last paragraph of Beaulieu's The Sea in Greek Imagination, and many gods and goddesses became associated with its safe passage (including Aphrodite and Dionysus). Sailors needed all the help they could get in those perilous waters, and who better than Ino-Leucothea, who jumped into the water only to become immortal? The Greeks, especially in later times, were fond of cultivating worship of mortals that had become gods by escaping death (or gods that somehow escaped the underworld, like Dionysus), hoping that in saving themselves, they held the key to unlocking salvation for other mortals.
Ino-Leucothea fulfills both of these criteria. Not only is she a sea goddess, and thus useful in helping sailors cross seas, she was a mortal who escaped death to become a goddess.
Because she wanted to help him. She just felt she had to intervene to make sure he was okay. She had compassion towards him and didn't want him to die so therefor she saved his life so she did not feel helpless.
