This question already has an answer here:
Someone may have answered a question and the external resources they referenced are no longer available. In this case I do not believe they deserve a down vote but I also don't like coming to a page looking for an answer to just find out it is no longer relevant.
The other use case I could think of is when you have significant technological changes and the suggestions are out of date. Someone coming to look for an answer might not know that the answer is out of date and trust the suggestions. It would be nice to have a flag saying that it was out of date.
This could also maybe feed into search optimizations so that out of date or now broken answers do not get top hits.