For the uninitiated, generally NSFW is for sexual contents and NSFL is for gory contents. People may want to see one but not see the other at any time for any reason. I have seen this feature requested over the years in Reddit but it never happens. Maybe now some instance can finally implement it?
Real answer? Because technology could easily do it for you if people tag posts in OP’s proposed way. Just one less thing for a viewer to have to think about.
It would also allow for different filters. Blur NSFW, but not nsfl, or vice versa. Or even only show post of one and hide others entirely, without having to block communities by hand.