After Merry and Pippin drank the Ent draught offered to them by Treebeard, they grew three inches to become the tallest Hobbits in history. The game designers have faithfully and thematically rendered the invigorating power of this mysterious drink with this card, which provides an excellent boost of 2 to any character, provided you control an Ent character who can administer the draught.
There are many potential uses for this extra health. Most commonly, you can boost your main defensive hero (or set up for undefended attacks on heroes like Glóin or the new Barliman Butterbur), letting them tank larger attacks or have some buffer to survive nasty shadow effects. But you can also use Ent Draught to great effect on Allies, too! A quick draught can greatly increase the lasting defensive power of one of your Ents (which you need to play the card in the first place), such as Derndingle Warrior or ally Treebeard himself. But it also goes well with other staple defensive allies like Jubayr, Defender of Rammas, Deeping Defender, or Eldahir so long as you have access to and an Ent. Outside of defense, the draught is very useful for the two Ent heroes, Treebeard and Quickbeam, both of whom need large health pools to power their ability. But the draught can also be used to keep squishy heroes or allies alive in the face of nasty treacheries like Blocking Wargs or Power in Their Terror. How many times have you wished your Warden of Healing or Henamarth Riversong had just one extra hitpoint to survive The Necromancer's Reach? Ent Draught can grant you the peace of mind to exhaust such allies whenever you need to!
There are many, many varied uses for this attachment. In fact, it's so versatile that you'll almost never be sad to have it in your hand... unless you don't have an Ent in play! So, how hard is it to meet this single condition? The answer is, not hard at all. You can always run an Ent hero to guarantee the draught, but it's far from necessary. Ents like Quickbeam, Treebeard, and Wandering Ent are easily splashable in any deck. Entmoot and Lore's native card draw abilities mean that even with only 4 or 5 Ent allies in the deck, you're likely to get an Ent out on time (and don't forget that Ent Draught has the Ent trait -- so Entmoot pulls this attachment, too!).
In conclusion, if you're playing and have room for a couple of Ents, it's almost always worth your while to throw in a couple Ent Draughts to help out whichever heroes or allies need a little extra health and survivability. Try it, it's refreshing!