I think it's more like a sign post in the text. At the start of any paragraph (or sentence, really) the text may go literally anywhere--could be a new thought, a continuation of an implicit list, an explanation of what came previous, or anything else.

If you say something weird or apparently unsupported, the savvy reader at that very moment is going to be thinking so. So it's helpful to orient them like:

> Here's a wild sentence. Here's why it's not actually that wild: reasons

Without the connecting phrase, the reader has to figure out from context that out of all the possible things the following text could be doing, what it's actually doing is explaining the previous claim.

You can rightly counterpoint that it's not strictly necessary, that a savvy reader can figure it out. But I think the moment right after a wild statement is a hotspot for readers getting ready to jettison, and having a little assurance is likely very helpful.

It turns out that both phrases are used like this, similarly to how they teach in logic classes that "but" is just "and" in fancy clothing, but actual usage is quite different. Actually, a lot of language is just signpost phrasing that "helps the medicine go down" by giving hints at how the following idea will connect to this one.

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Both phrases are used like this— let me explain:

Logic classes teach that "but" is just "and" in fancy clothing, and actual usage is quite different. A lot of language is signpost phrasing that "helps the medicine go down" by giving hints at how the following idea will connect to this one.