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, also saw a great uptivk in interest. The Sapling would soon follow, due to her utility in that sort of deck.
But, there's more to this card than simply her 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. Now, she's better than ever.
Also, the Sapling of Colfenor has quite a backstory that you can read about here.