In the movie "The Equalizer", why does McCall time himself with the stopwatch in a lot of scenes, what's the benefit of that?
Like when Nikolai showed up at his front door.
It's supposed to show that he is a master of time, probably as a mention/quality to his past life. Time here demonstrates attention to detail, habits and schedules. This is also seen in older movies like
or even more recently White Rose in Mr. Robot (eps1.7_wh1ter0se.m4v)
Time is their weapon and way to move about.
So, the constant references to McCall isn't a benefit, it's more of the meta-habit of the habits he executes. Very loosely it can be seen as obsessive compulsive, since he doesn't seem that bothered after killing the men in the room such that it didn't match the 16 seconds he initially predicted.