[ghc]

I recall the explanation of an excellent little Japanese grammar book that made it click for me: 私は puts the emphasis on what you, as opposed to everyone else is doing, and the closest english equivalent is "As for me,".

If you translate it that way, 「私は牛肉を食べます」means, in English: "As for me, I will eat beef."

Notice now that both "me" and "I" are present in the translation. So adding が doesn't in fact change the sentence at all!

[Tor3]

Yes, that's how e.g. Human Japanese (PC, Android etc) defines how the topic marker should be understood, and it definitely helps.