This Is Not Legal Advice. (See elsewhere in the thread for why lawyers can't offer "legal advice" to non-clients, even "legal advice that you should have someone else double-check")
I have no law education. When I read IAAL it sounds to me as some professional is about to give me some advice. So I don't really get it why anyone would write that on the internets.
This is how I read it: there is a difference between giving advice and stating facts/giving an opinion. IAAL informs that said opinion/fact has some weight but stating that it is not advice waives any liability as to what one ought to do with whatever statement has been made.
Compare with the following: I am a software engineer, here's my take on this or that technical matter, but don't apply this sample snippet as is in production, also I'm waiving all responsibility, like MIT-license style.
It's not that you shouldn't listen to random professional folks on the internet, it's that when they informally give some opinion on a matter, they may very well be missing some key part of your very specific context, and while common sense just would tell you not to take it as is and that you could hold them liable for anything, J. Random Bloke statistically lacks common sense so one has to spit proactive waiver statements telling people to act on their own responsibility or take advice in a proper, formal client-to-professional context.
_I am not your laywer_