The gimmick with these kind of brushes is that they have activated charcoal in the bristles. "Activated" in this case just means that the charcoal has been produced in a way that makes it much more porous and so able to absorb other substances than regular charcoal. Activated charcoal does have some medical uses - it's a standard part of treatment for various ingested poisons for example, as they can absorb and trap various harmful substances before they are absorbed by the digestive tract.
This has resulted in a recent trend for activated charcoal dental products (not just brushes, but also toothpaste, floss etc.), driven by, shall we say, the "alternative" crowd. It's not the first time we've seen these kind of real poisoning treatments get latched onto in the quest to get rid of those nebulous "toxins". There's a similar (and dangerous) type of woo that's grown up around EDTA, a standard treatment for heavy metal poisoning. At least activated charcoal is relatively harmless by comparison.
You're right charcoal can be abrasive though, although I suspect the amount that's embedded in these bristles is small enough that it'll have negligible effect (this also means it doesn't have enough volume to absorb any significant amount of those mysterious "toxins"). Eroding enamel is more of a concern with the toothpastes and so on, where your teeth are getting exposed to far larger amounts of fresh charcoal with every brush. As to any alleged benefits, there's at least one very dubious study claiming a modest reduction in bacteria around the gums, but I wouldn't place much value on it. Most of the claims made about the benefits of charcoal in dental products are completely unsupported scientifically.