I always understood it to be a caveat that the speaker does not have much or any formal legal education (much less in the specific state/country relevant to the conversation) and that nothing should be taken as actual legal advice by the reader, even if their situation seems similar.

Practicing law without a license is an offense in many jurisdictions; providing legal advice is in some cases within the scope of legal definitions of practice of law. (For people in certain roles outside of but around the legal profession, even implicitly holding oneself out as a lawyer or person qualified to practice law may have additional concerns.)

While public comment for which no one has been charged money may be outside of the bounds of what would be covered by most such laws, some people view it as best to be clear (and, in any case, such a disclaimer served as meta-legal advice that the matter at hand is one on which you should consult a lawyer if you have critical concerns.)

And, there are other concerned for lawyers in communications that might be mistaken for providing legal advice.

All that combined to justify disclaimers that include some or all of, as applicable:

I am not a lawyer.

I am not your lawyer.

This is not legal advice.

(Incidentally, I am not a lawyer, I am not your lawyer, and none of this is legal advice.)

It's an attempt to inoculate against the charge of practicing law without a license--which might seem incredibly unlikely, but that's how they got Ulysses Everett McGill.

But that doesn't require a disclaimer, if anything that just requires you to not claim to be a lawyer. Additionally: what recourse does someone have even if they read a random internet comment as legal advice. Pretty much zero.

Lay people can comment and give their opinions on matters of law. They just can't claim to be or represent themselves as a lawyer if they're not.

Of course that opinion is worth the paper it's printed on.

I am not a lawyer.