The fact that there is more text in a comment let's the reader know more explainin' is on the way. No need to point out that you're about to elaborate. Are people worried we'll get to the end of their paragraph and not know if the next paragraph is related?
It makes sense in face-to-face interactions, particularly when addressing are group. Sometimes, if you say something controversial, people will respond dismissively before you've had a chance to justify yourself. "Let me explain" is used as a way to reclaim the floor, so to speak.
For better or worse, people are transferring this pattern to written language as stylistic device.
http://www.hnsearch.com/search#request/all&q=%22let+me+e...
The fact that there is more text in a comment let's the reader know more explainin' is on the way. No need to point out that you're about to elaborate. Are people worried we'll get to the end of their paragraph and not know if the next paragraph is related?