Call me basic, but this is probably my favorite card in the game.
About a year ago, my group started playing single faction campaigns (all investigators are same faction). Now, Stand Together was already a pretty good card, but hoo boy - if everyone on your team is packing two copies, its power is amplified significantly. All four of us (Roland, Zoey, Mark, and Carolyn) spent our first 6XP from the Dunwich campiagn (Expert) on this card and never looked back.
This card is always helpful - and it's especially great on turn 1, as others have noted. But it's downright bonkers when you get to be the player of this card AND the beneficiary of a teammate's play of this card in the same round. If you are playing multiple Guardians, and whole team mulligans hard for this card, it's fairly common for everyone to have get a copy in their opening hand. You can increase your odds even further once you have Stick to the Plan in play to thin your deck (more on this in a minute). And of course, even if your teammates don't find it in their initial draw, you might be able to help them fish it out with your own play of Stand Together - teamwork makes the dream work!
If your whole team plays this on turn 1, then the net result is that each investigator spends one action to start the game with 8 cards and 9 resources (except for the Lead investigator, who will get their payoff from whoever takes their turn last). For our resource-hungry Guardians, this is a great way to hit the ground running.
Probably the most noteworthy combo is with Ever Vigilant. After a few scenarios, you'll likely have it tucked away under Stick to the Plan, but even before that, you'll have a reasonable chance of drawing it, even if you only have one copy. With all of the extra cards and resources at your disposal, it's much easier to maximize its efficiency as you are much more likely to have three playable assets in your hand. There's nothing quite as satisfying as having your Seeker-focused Roland play Dr. Milan Christopher, Flashlight, and Magnifying Glass for four resources with one action. After Zoey and Mark are similarly set up, you'll almost feel sorry for any enemy unlucky enough to spawn at your location. Almost.
Another benefit to such early drawing is that a lot of Guardians like to draw their weaknesses as early as possible early, and this opening gambit helps you make that happen.
Sometimes it's just about the simple things.