Seething Chaos is actually pretty good depending on how you manage its utility. But simply put... AREA CONTROL.
Obviously if someone knows what the spell does you'll more than likely never hit them. And this is actually fine, its not hitting your enemy that this spell is necessarily for. But if not, what purpose does it have? The following:
- Throw Seething Chaos and move to the immediate proximity of the ticking time bomb for another spell cast. I personally take advantage of this to cover for the use of Floating Chaos, or oftentimes two if my opponent doesn't have any ranged weapons or items in use. This allows me to see a potential movement pattern they take and then bomb the path of said pattern with Chaos Fire Orbs. Works surprisingly often and oftentimes when I do hit, they also get hit by one of the Floating Chaos Shots for additional damage alongside the lava. Then there's the other side of the fence...
- Naturally if Seething Chaos can be used to support other damage spells it will also work in tandem with any other type of spell... For example, one that instead of targeting your opponent's HP, brings your own back. Setting up a Warmth is something lots of people are familiar with by now thanks to the Lazy Pyromancer bandwagon born on Youtube, but there's a way more effective/potentially cancerous spell on the market. That being Projected Heal.
Now I haven't done this much myself since I prefer using offensive magic and my attunement slots are all occupied for that purpose, but Seething Chaos is one of those spells that people see and go "welp, staying away from that one" since the explosion deals A LOT of damage, more so than Chaos Bed Vestiges for comparison's sake. There's no reason to risk getting caught by something of that magnitude even if there is a 4 second interval in between the Spell latching onto a surface and the actual explosion. With this in mind, you can time Projected Heal for the latter two seconds of the countdown. Approaching you in the midst of this is suicide.
If your opponent happens to be a melee focused build there is just nothing they can do. If they rush in they get caught by the explosion and with Unfaltering Prayer in use your heal won't get interrupted by their attack anyway. On the other hand, if they choose to throw something at you instead it won't be enough to offset the HP you just gained. The only true counter to this are other schools of magic, like Sorcery for one as the damage is high enough to negate what you would've gained.
How other players use this is entirely up to themselves, but I feel like I just presented a few examples of why some people are too quick to judge spells that don't follow the pattern of cast = damage. Utility can be just as, or even more so powerful than direct damage.