
When people write either of those two phrases or some similar variation of them in an article or essay or blog post or whatever, when the whole point of whatever they're writing explicitly is to explain the thing they're writing about... I don't know if this is just the Baader Meinhof phenomenon or what, but I've seen it quite a lot in just the past month or two (though I've definitely seen it well before that, too), and it has bugged me every time. You really do not need to write "let me explain" in your thing when you make some assertion somewhere in the first paragraph or two of it. You really don't. I already assume you're going to explain, considering that I can clearly see that there are several more paragraphs after that in your thing.
The only thing I can think of is that it has somehow become shorthand for "I know I just made a potentially ever so slightly off kilter statement and I want you to know that I am fully aware that it may seem off kilter but that it may not be as off kilter as you think it is, and now I'm going to tell you why." Still don't think it needs a lampshade hung on it, though.
(I'm talking only about when those phrases are used in writing, not in verbal conversation.)