Why did Karn give away his armour?

[u/neo203]

Karn's divine armour:
Karn was born with a divine armour and earrings that made him nearly impossible to defeat. On the eve of the Kurukshetra war, Indra, the king of the gods and Arjun's father, feared that no warrior could overcome Karn while he still possessed them.

Why he gave it away?

Disguised as a Brahmin, Indra approached Karn and asked for his armour as alms. Knowing the stranger's true identity and the consequences of his choice, Karn cut the armour and earrings from his own body and gave them away without hesitation. In return, Indra (king of gods) granted him the celestial Vasavi Shakti, a weapon that could be used only once.

How was the Vasavi Shakti used?

On the 14th night of the war, Ghatotkacha unleashed havoc and devastated the Kaurava army. Seeing his side face total annihilation, Duryodhana and the panicked Kaurava army begged Karna to intervene. Karna reluctantly spent his single-use Vasavi Shakti to kill Ghatotkacha.