Well, unwarranted advice can result in making someone feel patronised, or like their privacy or personal boundaries are being violated, or like their personal circumstances are subject to public speculation, and these are all unpleasant and negative experiences, and you should try and avoid subjecting people to them.

It can also, out of nowhere, create a whole raft of dubious questioning or accidental insinuation that the recipient of the advice may feel obliged, or even compelled, to put straight. It has a general capacity to generate discussion that is a lot more effort for the advisee to engage with than the advisor. It's very easy to give people advice, but as I have found, it's surprisingly hard to say "no, stop, I don't want this advice!" (I have said it very vehemently in this thread, with the consequence of looking like an objectionable arse, but I'm not sure that saying it less vehemently would have actually stopped people from offering it.) These are also unpleasant and negative experiences, and you should try and avoid subjecting people to them as well.