What should it for?
Note that we are not health professional experts, so treatment and what might happen are both off topic.
My opinion is that it's a valid tag relating to how to work around injuries. Ideally, you could argue that the person should wait until they are completely well before returning to martial arts, but time spent not training is time during which you may be actively getting worse at your martial art. To give an example of a relevant question, there was one on the Fitness SE about recovering from a sprained wrist, specifically on how to get back to doing pushups where I pointed out that I'd found that knuckle pushups are easier on the wrist in that situation, letting you get back to them earlier. Similarly, I could see someone suggesting alternate Capoeira movements when dealing with a bum knee, or chair self-defense exercises when you want to practice Eskrima after you've tweaked your back (both situations I've dealt with and gotten advice on in the past).
I'm of two minds of it.
On one hand, yes, you don't want to devolve into medical advice online. A lot of things need to be assessed in person by qualified folks to say what's going on with any certainty.
On the other hand... the fact is that training itself is a form of rehabilitation - the same science that teaches you how to strengthen muscles or develop coordination is also the same science you're going to use after your ankle recovers from a sprain.
An injury often lasts far beyond the time it takes to heal - aside from scar tissue, the bigger concern is the increased chance for re-injury. The 3-12 months after an injury is often where athletes are at big risk of re-injury, simply because the tissue is still trying to recover and they are pushing it when, post injury, the answer might be having to make shifts in how you do a particular thing, or specialized exercise aimed at strengthening your stabilizers to avoid that.
These questions are part and parcel of martial arts training. Obviously things like "I've broken my arm, it's not working" is "Go to the hospital" kind of questions, but the longer term issues are things every martial artist has to deal with and somewhat critical to talking about training, itself.
I think recovery and prevention questions are probably going to be very useful and, cannot be worse than anything else online people could be getting at other sites, especially since the vote system provides some measure of quality control.