So as @Nuloen The Seeker pointed out, Baba Yaga has a role as gardian of the dead and the underworld
(The french Wikipedia seems to be the most complete one on Baba Yaga, maybe because more of Vladimir Propp work were translated in French).
There seems to be a possible association between Baba Yaga and Morana the goddess of winter, found on Slavorum:
In ancient times people especially worshiped Morena in the territory of Kievan Rus. Initially, she was the goddess that was worshiped as a homemaker, but also the goddess of fertility and mother of all living. However with time gradually Morena transformed into the evil messenger of wilting, impotent old age and disease. It should be noted that in many ancient tales she confronts Perun and sometimes even joins Veles in mischief and trickery.
They seem to have a lot of caracteristics in common:
- Old age
- Sometimes achieving mischief and trickery (Baba Yaga is evil almost the two third of the fairy tales where she appears)
By going a bit down in the article, you can also clearly read:
In some legends it is said that BabaYaga is also one of the creations of mistress Morena, or even that BabaYaga (Baba Roga among south Slavs) is just one of the transformations of Morena.
Now lets present the infamous
His immortality is described as follows:
Most seen Koshchei as an immortal being and one of the fairytales opens up with him saying “My death is far away: the sea on the ocean there is an island on the island of oak stands under an oak chest buried in the chest – a hare, in a hare – a duck in a duck – egg and egg – death my”. Many scholars have seen in this “matryoshka” interpretation model of the universe: water (sea, ocean), earth (the island), plants (oak), animals (rabbit), poultry (duck), and oak – “world tree”. In other words, you can only kill Koshchei by destorying the world itself.
He was seen as an evil, powerful, being:
Still, in various Russian myths Koshchei is seen as a powerful frost sorcerer, with cunning mind and not so good intentions. In few different fairy tales he turns people into a walnut, or even turns entire kingdoms into stone so folk legends still see him as an evil character. As a old cunning sorcerer in fairy tales he is usually bound to build his activity around young pretty girls where he tries to seduce them and win their love with magic or even brute force. In his failure and rage he even curses these young girls in animal form to become frogs, snakes and other wild life.
So now it brings us there:
Why would Baba Yaga want Koshchei heart?
One of the obvious reasons would be to gain control over the powerful and immortal Koshchei.
Another would be to stop his nefarious actions and maybe even send him to the Underworld to restore the worlds balance (Yeah I still think immortality is OP). There's this story about him and Ivan Tesarevich, where Ivan found him and helped him recover by breaking his wife, Maya, trust:
As days passed Ivan was curious and he entered the closet even thought his wife Marya told him not to. In this closet he found an old man chain-locked to the wall by twelve chains. Old man was weak and in terrible condition and he begged Ivan for water and food, or he would die there. Ivan being good hearthed gave him a few barrels of water. Just few moments after drinking it, the old men started to restore himself and suddenly it was clear, he revealed his true form, it was Koshchei the Deathless. Now fully restored and replenished Koschei was strong enough to break free of the wall chains and in anger he said he will kidnap Marya.
After a lot of adventures, deaths and resurrections, Ivan encounters Baba Yaga that gives him a horse faster than the one Koschei owns:
After successfully completing few tasks for Baba Yaga he was awarded with the fast stead. With the new horse he was finally able to save Marya and escape Koshchei once and for all!
So we can imagine Baba Yaga would want to avoid the nefarious action of Koshchei. But then, we can see in the end of the page about Morana:
The Eastern Slavs also believed that Morena was the wife of Koshchei the Deathless, who loved to make mischief to people.
So the actual reason to getting the heart can also be to gain control over her husband and maybe stopping his actions without hurting him.