So while many people will tell you this is a stupid question, I think it's perfectly valid and I also had a chloe and did not really have a good use for her for a very long time. Let me try to drop an answer here.
First, chloe is dependent only on speed and hp. Speed first, then hp. Why? Primarily arena. Presumably you have seen a bunch of teams that use megan, lushen, tyron etc. These teams goals are usually to nuke down your team or keep them frozen/stunned long enough to kill a couple of them and screw you over before you even have a chance to do anything. This is a pretty effective strategy. BUT if you have a chloe with high speed who goes first, you pop her third skill and your team is untouchable for a turn.
Many a time have I run chloe's third, then had megan buff and increase attack bar, followed by tyron and lushen's third skills, then your team comes out entirely unscathed and wrecks them. When this happens it is pure magic and you immediately understand why chloe is so good. She simply hard counters DPS teams in arena. On top of that, the immunity last for 2 turns, so even if it takes the opposing team a couple turns to run their CC skills, you are still covered.
The caveat is that she needs to be really fast. The other caveat is that on top of this, you want her to have really high HP. That means swift set no question, and on top of that you want a 6* speed rune, and at least +10 speed subs on all your other runes if possible. Oh, and your 4 and 6 slot need to be strong HP% runes and still preferably have good speed subs. BUT if you can get this kind of rune set together, and on top of that have her skills maxed, chloe becomes an absolute monster. On top of that, once her HP starts going up over 20k, you start to realize that she hits really hard as well, since her first skill damage scales with HP. So she's not just a support, she actually packs a decent punch damage-wise.
So once you have Chloe runed right, we're talking about a huge tank that hits fairly hard, and also hard counters DPS arena teams, frequently gives your team immunity, and has a pretty solid heal as well. That's an extraordinarily strong unit. But you need to have the runes to build her well to get there. Personally I had chloe sitting around useless for months before I got the runes I needed to make her shine. But now that I am closer to that, she is really coming through as a staple of most teams.