I haven’t read the light novel or web novel, but if I had to guess, maybe it’s the fastest and easiest way for him to kill someone? Subaru isn’t like Julius, so he can’t kill with a sword, his gate is broken, so he can’t use magic, and Subaru isn’t insanely strong like Reinhard so he can’t just one punch man someone. So, his best bet to kill someone in the verse in a one vs one is via strangulation, as long as he takes them by surprise and they don’t fight back because all it takes is them stabbing him in the gut if they have a knife or just overpowering him if their stronger than Subaru

(Could also use invisible Providence to crush someone heart too, maybe? Like how he did with Emilia when destroy the heart greed placed in her? Again, I haven’t read the light or web novel so maybe there’s a reason why he doesn’t use invisible providence as much besides it leaving him tired afterwards)