Nano boost does not prevent you from doing anything else for the duration of the boost. You can nano boost someone and continue doing other things that have a direct impact based on your own game play, you could kill someone, save someones life or sleep an ulting genji if you are skilled enough all while nano boost is going. Speed boost is the same.
And you have control in that you can choose who to boost. That is all the nuance to damage boost, who I right click?
What they do is entirely beyond your control and for as long as you boost them you are tethered to them and unable to do anything on your own unless you choose to end the boost.
You cant even necessarily see what they are trying to do because you are most likely and should be behind cover or paying attention to the rest of the team/ team fight.
I’m sure you have been in the situation where both teams are just poking and you are boosting someone to try and give them the advantage. Most likely you are just standing there, possibly a/d spamming while you wait for something to happen.
How are YOU as a player really impacting the flow of the game at that point. The impact is deservedly on the player you are boosting for their mechanical skill at their dps hero for getting the pick or the damage in.