IANAL (ya see what I did there?), but probably not necessary as a legal requirement. I always viewed it as “juuust in case that you think I’m a lawyer offering legal advice because I almost sound like I know what I’m talking about, I am neither a lawyer, nor do I know what I’m talking about” courtesy to the reader.

I cannot imagine getting pulled into court because I recommended someone file in small claims court for $PROBLEM, and didn’t put “IANAL”.