Without the disclaimer, in some jurisdictions the post could be interpreted as unauthorized or inappropriate practice of law. http://en.wikipedia.org/wiki/IANAL
A real "legal advice" is a professional advice, as is ascertained by the board, which gave license to the person (lawyer), who is qualified to dispense such legal advice. Giving a "legal advice" when the giver is not licensed to give such advice is dangerous to public interest, and is discouraged through laws and customs.
Now, any person can give any other person an advice on legal matters, as long as the target of the advice is not fooled into thinking that he got the real certified stuff. To stay on the right side of the customs the easiest thing to do is to confess to lack of credentials and make things abundantly clear by directing the other person to the real professional after expressing your initial concern or opinion.
Now, it might seem backwards to you, and you might expect that the default would be "people talk shit all the time, so no one should listen unless the speaker actually provides credentials". This is how it works in most areas, but not all. In particular law and health are two areas where the state saw it fit to go out of the way to protect the least savvy members of society by twisting the default setting the other way around.
Obviously, this is not a legal advice on how to give legal advices, or any other matter.
IANAL, but I think that in this context "legal advice" has a meaning beyond the English one. It is also used to indicate an attorney-client relationship and the associated liability and confidentiality.
So someone can give legal advice (English), but say it isn't legal advice (Legalese) and that makes it not legal advice (Legalese).
The main reason for IANAL, TINLA is that lawyers have attorney's immunity, and joe shmoe does not. If somebody relies on my opinion as if it is legal advice and from the circumstances it looks like legal advice, I can be in deep poop if the advice was wrong.
