The Ashta-Vasus were cursed by Vashishta to be born as humans because they stole cow Kamadhenu. 7 of them were spared by a quick death after birth, but Prabhasa alone would have to live a long, painful, illustrious life as Bhishma.
Many of us face suffering in life. Most of these times, we know the cause of suffering (our own karma) - whether it is clearly self-inflicted, or it comes through the disguise of others. But sometimes, we suffer additionally because don't know the cause, and wonder if it is 'Prarabdha' - an unknown consequence of something we did in one of our innumerable past lives. If we know the cause of suffering, it definitely eases our pain a bit. If we are punished for a wrong-doing, at least we'll know that we deserved it and can also avoid that mistake again.
Did Bhishma know that he was born on earth due to his stealing in previous life?
We know that Bhagavan takes avatara as Rama, Krishna and that he is fully aware of who he is, even though he may choose to hide or display it as he sees fit.
But what about other 'avataras', of devas / asuras ? Some people do know the cause e.g. Nahusha knew he had to be snake due to Agastya curse. Did Shantanu know he was born due to curse in Brahma Loka ? Was Bhima aware he was Vayu-amsha-avatara ? etc. Same goes for other characters in Ramayana/Mahabharata..