Guardians thematically stands guard at the same location as the Seekers to help with spawned enemies, but that is not enough as you still have to spend Engage action to grab it if you are not the one that draws the enemy. Guardian also loves to dispose the enemy quick before its first Enemy Phase arrives, so 3 actions are not to be wasted.
"Until the end of phase" split this card to 2 use cases : Investigation Phase or Enemy Phase.
For Investigation Phase usage, this card buff all your other friends at Mythos Phase with something that feels like this very expensive Ward of Protection (5) or Disc of Itzamna (3) that could kill an enemy Mythos draw. They can go first in turn before the fighters, continue using Investigate/any business they want to do, provoking AoO, then the enemy is instantly forwarded to the Guardian fighter at good deal of probably 1 health each. Guardians love 0 horror spill (including the guardian dog). Guardian go the last, then use full 3 actions to take it out. Not only you save actions, you also gain back turn order flexibility.
Enemy Phase usage is rarer. You can defend against incoming Hunter enemy, and is a play of Prey criteria :
- If Hunter but no Prey : No need for this card if you decided to stay, as you can already select engagement target.
- If Hunter and has Prey that is not you : Stand guard and let it walk in. It engages with Prey instead of you, then play this card. Though, without Get Behind Me, Guardian can instead walk forward to it and take free engagement instead of standing guard and fight (i.e. standing in its way). Downside of doing that is you can no longer receive commits from your Seeker friend and also lose an action to walk. Get Behind Me still has good value opting to stand guard strategy against Prey enemy.
- If Hunter and has Prey Only that is not you : It is almost same situation as Prey but according to the rule the Guardian no longer able to walk forward and take free engagement, you must additionally waste Engage action. Resulting in 2 wasted actions before the fight, which you may not be able to take it out. In this case, Get Behind Me can save you that 1 Engage action in addition to all other benefits of just standing ground.