>>/28082/
That's basically it. It's not to 'dox' anons (it doesn't do that as the auto-assigned UserID is just a combo of letters/numbers that have no attachment to your actual person). Sometimes someone accidentally replies tot he wrong post, or some glitch happens (I've had both) and it gets almost hopelessly confusing.
It's really just to eliminate confusion. It's the way other boards are on chans AFAIK (even this chan), so this is an anomaly. it's frustrating and confusing without UserID per thread.