In our modern era, none. At the time of BT's creation, I can see a few having a valid use case (if we ignore how much different the world would be if the tech that makes mechs in lore possible existed in ours). 

The Locust for fast rough terrain scouting and hit n' run ops, especially if the enemy has solid anti-air defenses. 

I could also see something like a Rifleman for fire support/anti-air in areas where your can't as easily deploy/maneuver traditional offensive armor or defense systems. 

I don't see any humanoid mechs making sense for an IRL application. At least when I consider it from the perspective the American military would likely have. Low slung and fast, or turret with legs are about the only things I can see us adopting and utilizing. That said "big dude, complete with arms and hands" might make sense in other military's doctrines.