Single action to "Protect". Help me with balance

Hello there, Pathfinders of Reddit!

My name is Drakonyan and I am a professional Game Master. My campaigns are homebrew and I love to house rule elements that help me tell the stories I want to tell. But I also worry a lot about balance and try to always understand the R&D choices the developers of an RPG system make.

More recently, I made a house rule to create a new basic action: Protect.

It costs 1 action. You give an ally within reach a +1 circumstance bonus to AC against enemies that you can also reach.

You don't need a shield or even a weapon, since all that you need to be able to do is get in the way of the enemy. If you have a reach weapon, then you may protect against more enemies, but you still just give a specific ally a +1.

I need to know if this is fair and balanced. I don't expect this mechanic to be much used in later levels, when everyone has multiple things they can do with a single action. But for low level style of play, which is more tactical and with more limited options, this seems very handy in the hands of my players. Yet I can't stop thinking: "they would have thought about making this a core part of the game, wouldn't they? I must be missing something".

Well... am I missing something here? Is this fair? Is this just a bad use of an action? What do you guys think?