Uses of Subtitles:

• Easier for weaker English speakers to understand. I started doing them because an Eastern-European suggested subtitles some years back.
• English subs make a good starting point for foreigners to make subtitles for their language (*) (mostly in having the times worked out). The young people of foreign countries tend not to learn English until later (though MM is not really meant for younger children anyway, and it'll be a sad day when we have to teach them about European Identity really early).
• Convenience, for when you want to watch in silence for some reason.
• For the deaf.
• Call me immodest, but imo it adds entertainment value. Punctuation can help with expressivity and interest, when all you've got is a mouth animation to look at. Or, when it's long + complex, but you're supposed to understand it, like Terence McKenna's explanation of the growth of complexity in 'Guardian of the Rune'.
• Good for making /pol/ .gifs / .webms: turn subtitles on, play and capture your clip, turn into a .gif / .webm.
• Would help with making printed 'comics' for /our/ political prisoners (I admit I've never done this - I'm afraid I'd be put on more lists, if I mailed the result under my own name. And, it would be quite a job, and it isn't certain that the prison people would let it through).
• It's pretty easy, and satisfyingly semi-creative fun. (Creative, because you have choices as to how to punctuate, and how to break up utterances, and you have to feel what works best.)

Howto:

I use a machine transcription service and some scripts to generate rough starter .srt (subtitle) files which cuts the work by 85% or more.
The starter helps with times and 90% of the words, but is far from perfect. Since you'll need to re-group some words, I also generate a 'helper.txt' which tells the start and stop time of each word, which in turn will tell you the start + stop times of any new line you make.
VLC permits a nice workflow (Subtitles / Add File...), if you don't already have a video player that lets you load subtitle files.

I will credit people who help with these, on the About page.

(*) A non-native speaker can do it too, with Google translate: watch out for English idioms, try to re-state them - if the translated version comes out similarly enough to the original, well, the translator can handle idioms - great! If not, choose the translated version of your restatement, especially if it uses different words. If it's a clunker in the destination language, well, at least the meaning should be clear. I have done this once, though not for Murdoch Murdoch. You couldn't expect to be as good as a native though, and especially for slang- and reference-filled stuff like MM.