The main drawback to Forest Snare is you usually have to take an attack from an enemy before you can play it on them. This is because it can attachments can only be played during the Planning phase. Engagement happens a few phases later in the turn. Players then have to engage the enemy during the engagement phase, endure the attack in the following phase., and finally next turn can snare the enemy.
Son of Arnor fixes all of that. He lets the player engage an enemy when he enters play. Note this doesn’t have to be in the staging area. It can be an enemy engaged with another player. Still to avoid an attack ideally you’re pulling an enemy down during planning to immediately put Forest Snare on it. You can do this even more cheaply with Sneak Attack to put the Son of Arnor into play, engage an enemy, and then snare it.
This is fantastic tech for Journey Along the Anduin for the Hill Troll. The troll’s 6 will outright kill most heroes in the core set even without shadows. Chump blocking with an ally isn’t ideal since any excess damage becomes threat. Staying low on threat, then using Son of Arnor to engage the troll and snare it avoids any threat gain or potentially dead heroes. The Troll then can be dealt with over several turns as the player builds up their board state. The player(s) can basically choose when to advance to Stage 2 when they’re ready for it.