The Bloodied conditions on its own is really just that keeping score on whether you're at half hit points or less or not.2. Bloodied. I basically started using this because I had players keep asking me if an opponent looked bloodied. They basically explained it as half or fewer hit points. I didn't like it at first, but it grew on me. It is a simple way to telegraph how injured opponents are. I mean, it is fun to try to go all Matt Mercer and describe in detail the condition of an enemy, but it quickly get repetitive even if you are fairly creative and good at improv, especially if there a large number of combatants. At some point, saying someone is "bloodied" just keeps things moving. But it always bothered me that there was no mechanical significance and I'd like to read more about how the bloodied condition (if it was a condition) worked in 4e.
But because it's a condition explicitely called out in the game mechanics, other abilities can key off of it.
You could have a necromantic spell or assassin ability that deals extra damage if the target is bloodied.
You can have a Barbarian get more dangerous when he's bloodied.
You could have some divine abjuration spell that gives you protection only when you're not bloodied (probably something about Grace and looking like a God) or when you're bloodied (probably something about having the Gods aid you)
You can have a monster that launches a powerful attack when it gets bloodied.
You can have a feat that gives you extra damage against a bloodied foes.
It is a nice mix of game mechanic and (combat) story telling tool, basically. Because you're so hurt, your enemy is smelling your weakness. Because you seriously hurt the enemy, he's pulling out all the stops. Because you seriously hurt the enemy, he is discouraged and changes to a defensive tactic or retreat. Because you're seriously hurt, you don't appear as unbeatable as before.
Even without any additional mechanics, it gives you a pacing mechanic and a way to tell how the combat is going, without needing to explicitely give the player hit points values or anything. "Two of you are bloodied, but none of the enemies are? This isn't going well, consider a retreat or taking out the big guns". "That enemy is bloodied, we should focus on him to take him out". "Your friend is seriously hurt, you should heal him!"