It's basically a guard from Metal Gear Solid, except it can't attack, or hear your footsteps, or smell your cigar if you happen to be playing playing Graves. I don't think it can detect things that are already stealthed, either. Still, it's surprisingly useful.
It still lets the whole map know you done goofed if it catches you, and it's probably harder to evade than a stationary ward in some instances, and unlike in the Metal Gear series, there's no way to take it out without alerting the enemy.
At worst, the fact that it went dark early means a gank is incoming along or near its route. At best, its presence forces an enemy jungler to evade it, and thus gank in a manner which a canny team can predict and counter.
Also, it can be used offensively to provide vision for long-range spells/skills.
And of course, its passive encourages some nifty comps and combos by taking a lot of offensive pressure off of the support role. The potential of proc'ing Concussive Blows and Soul Bound at the same time is a sobering thought. Braum and other CC tanks also help Kalista's ult by not only surviving it, but in effect prolonging it.
Going off on a tangent here, but Braum and Kalista look like they go together, like spear and shield, one might say. Braum's slows and stuns would let Kalista kite and stack for Rend, and Soul Bound and Rend could deal enough damage that Braum would just need to land Concussive Blows/CC and defend with Unbreakable/Stand Behind Me.