This makes the Sapling a great way to get creatures into your hand easily while gaining more life than you'll likely ever lose by doing so.. Also, because the Sapling is indestructible, you can swing away with her without many worries. (only effects that can exile it when it's attacking are a problem). Therefore, the Sapling's effect should activate fairly often.
With the introduction of Assault Formation in Dragons of Tarkir, copies of Doran the Siege Tower more than doubled in price. That Enchantment essentially doubles as a second copy of Doran. Forgotten Ancient, another popular creature in that deck, has also see an uptick in price. The Sapling is the latest to see an increase in price, due to her utility in that sort of deck.
(Also, Sidisi, Undead Vizier, introduced in Dragons of Tarkir as well, is a 4/6 with deathtouch that both works well with Doran and Assault Formation's effect and also serves as a sort of Demonic Tutor. This Sidisi works well with both Doran and Sapling of Colfenor.).
But there's more to this card than simply its strong interaction with Doran & Friends. Being a Legendary Creature of her own, the Sapling can support a similar kind of deck of its own with the addition of Assault Formation. The Sapling has long been used as a Treefolk Tribal Commander, and now she's better than ever.
Also, the Sapling of Colfenor has quite a backstory that you can read about here.