I would say it means you helped him somewhat above and beyond the call of duty, or that you provided the needed help even though he feared that you might not be able or willing to do so. Anyway, it is a big thank you.
From your user name can I take it that you’re not a native speaker? (Honestly just curious–it’s interesting to me when someone isn’t familiar with an expression which seems like common knowledge to me. I like to find out the story in such cases.)
As far as the bigger, more literal reason for using that phrase, as in ‘came through’ what exactly?, that might be one for Cecil.
But I can imagine it’s similar to the expression, “Came through that unscathed” which simply means you endured a challenge or ordeal without any injury. Again, either in a literal sense as in a soldier going thru combat, or something much less like a shy person at a party. So to ‘come through’ for someone else just means you *‘entered into’ *some type of challenge (maybe big but usually small) and *‘came out through the other side’ *successfully.
Yes, it’s similar in that they both are phrasal verbs, but the OED lists that as a distinct definition:
The meaning from the OP is a second, different definition:
A third definition has to do with perception or appearance:
I’ve always maintained that the hardest thing to master in English are the phrasal verbs, because the same combination of verb and particle can take on various distinct meanings, and because of their essentially arbitrary syntactic restrictions. However, this challenge in English is fortuitously not necessary to use English with a certain degree of effectiveness, because most phrasals have some corresponding verb that carries the same denotation:
They came through unscathed => They survived unscathed
My friend came through with the money => My friend produced the money.
Her attractive personality always comes through when she presents. => Her attractive personality always emerges when she presents.
One way that native speakers unconsciously perceive non-native speech is by the “avoidance” of phrasal verbs in preference to such (usually Latin origin) forms. Someone who otherwise is quite fluent, has excellent pronunciation, and otherwise uses a rich vocabulary will still give off a non-native “impression” by avoiding phrasal verbs where native speakers would tend to use them.
I’m with guizot’s “emerge” understanding. In other words:
I needed help. You could could have stayed isolationist and done nothing. But you went above and beyond the call of duty. You broke out of your shell and emerged to help me. You really came through for me.
I have always thought of the phrase as referencing a siege or other military group cut off from the rest. I’m not saying this is the official origin in any way, but if I say “You really came through for me” I’m meaning it a lot more than “You emerged/stood out from others” or “You were successful in helping me.” If I say it, I mean it in the “You risked your life to break through the enemy lines to save me.” (Which, I suppose, is why I use the phrase rarely and never in a casual sense.)
I think it means a little more that some are saying. To me it means he was really counting on you and you really helped to make it all work. It also suggests that he owns the activity and that you were an essential helper that helped him to boost his own station. . . .YOU came through for ME. . . it’s really all about me. But, that may not have been intentional. . . . basically, it’s like saying “you did a good job for me.” or “you really helped me a great deal.”