I used to watch movies on my laptop, and I've not needed subtitles after getting comfortable with English. But now I have a proper speaker setup (stereo though) and the fact is that the mixing can be horrendous. Someone is talking normally and then – BRA-AH - some audio effects or shooting happens, and you miss part of the conversation. And some accents are just about unintelligible for me. So, I just add subtitles when I want to follow the conversation.