Oh boy I hate to be considered vague,
Where do we start. If I get your back on the street and want to strike I can strike at some of the following plethora of targets :
Back of Head
Back of Neck
Spine (all the way from the neck on down)
Kidneys (ever seen someone dropped from a kidney shot it happens like lightning)
Underneath Buttocks to Groin
Back of Thigh
Knees
Calf
Ankles etc. (remember most people probably have a tool, at least I do. Oh and I would definately break their balance and base before I started striking. (if I could that is)
Okay all of these places can be targeted emptyhanded but since I am a tool based person if I have someones back then I am going to be using a tool to amplify a strike. (like a stick, knife, force amplifier)
So those are just some of the areas that could be striked and that does not include if you decide to bend the guy over backwards, or strike from the side, etc. (lots of targets)
Then we are simply into maybe just taking the guy down by locking up a leg from the side or blowing out his knee from the back. What about a rear double leg takedown or a single leg takedown using forearm or shoulder. There are more ways to get them down from here. (that is for sure)
Never forget though the simply and genuinely nice way of a standing or prostrate rear naked choke. (that would be my preferred method, just putting them to sleep:rofl
I have used this technique more than once from behind to handcuff someone: Forearm across Throat and opponents arm behind their back to break their balance with my foot then going to the back of their knee to bring them backwards and down. Roll them on their belly and begin the handcuffing procedure. (very effective)
Bottom line if you can break their balance you can then do multple things like control, takedown, strike. Lots of options if you do it right!
The list from behind is a lot longer than this and your opponent is in a very vulnerable position because he/she cannot see you or what you are going to do.
Your big tools are ready to go and the opponent is vulnerable. So if you can be behind them good for you!
I hope this is a little less vague and a little more helpful.