What is the use of the Nobushi's Hidden Stance?

I found a pretty good summary and when-to-use guide for Nobushi's Hidden Stance in this thread:

Entering Hidden Stance gives you a small amount of I-frames, as if you were dodging an attack. While in hidden stance, your stance indicator disappears, so your opponent cannot see your stance until you leave the Hidden Stance (which happens when you switch to an actual guard). Entering Hidden Stance will cancel almost any action (similar to the Conqueror's onmiguard), making it a great way to feint attacks and mix up combos. You can also do the unblockable kick move straight from Hidden Stance by pressing the GB button.

Basically, her Hidden Stance can be strategically used as a quick dodge or a way to trick opponents, since it cancels out most actions.

There is an entire Nobushi guide that comes up too.

From there specifically on the Hidden Stance:

For Honor Nobushi Hidden Stance

With Nobushi, if you guard down, you’ll enter the Hidden Stance which allows you to avoid any incoming attack with proper timing. The exceptions to this including feats and guard break.

However, the timing is little earlier than parry so you may be required to use it predictively. You can also hold the stance to confuse the opposing heroes as to what will you next.

When in this animation, you can go in with a kick and surprise the opposing heroes. However, do note that this ability takes a good chunk of stamina and halts the stamina regeneration for a while.

So on top of what Vemonus said, it is worth noting that the things it will not stop are feats and guard break. Also that in a strategic sense it is good for opening fights, as you can start out by not giving away your positioning or stance, which can be quite advantageous.

Just adding to the previous answers, Hidden stance also counts as the first move in the 3 light attack combo, so instead of having to light attack 3 times to get the bleed effect, you only need to press it twice from hidden stance.