They have the same meaning. You can also say "Let me know if you need any help". Therein lies would usually be written or may be used in formal speech. You are right in that "therein lies" highlights the thing that came before it. It explains how the thing coming before it effects the situation described. It is often used to indicate when something causes or solves a problem. This is famously quoted in Hamlet "therein lies the rub" meaning "that is the problem". For example you may say "I had no money and could not buy the sweater. I had paid my rent and was broke and there in lies my problem" - meaning my problem is I had no money Or "I wanted to go to the party with my friend, but my other friend wanted to hang out with me also. I realized we could all go to the party and therein lies the solution to my problem." OR "What does two plus two equal I wondered. Then I realized two plus two equals four. Ah, the number four, therein lies the answer to the math problem I was wondering about."