Similar to how the Spoiler tag is used, only specific boards like /a/ and /vg/ would allow you to use the NSFW tag. To prevent it from being used as a free excuse to post porn in threads the same on blue board rules would apply for things posted under it. For example, if an anime/manga in /a/ or game in /vg/ had some really lewd or questionable content in it, users would be able to post about it using the NSFW tag and discuss it in detail without risk of a ban. If anyone just randomly went off topic and started posting porn (obviously unofficial stuff) they'd be viable for a ban. This would help when posting about the more hentai-ish content in the Fate series, as well as the extremely ecchi anime that tend to push the limits with what they can show. Right now we can't even post self-censored or cropped content in /hgg/ or official /a/ content because mods will quickly ban us for it without question. The NSFW tag or something similar to it would help improve the user experience in those specific boards.
Boards like /v/ wouldn't have it for obvious reasons.