Sometimes the question is tricky and the person answering it assumed certain reasonable conditions. The answer may still be useful for other people having the problem but is recognised as not the answer.
For example consider this question - the answer may still be useful for people searching but it does not solve particular problem (which is about Windows 7 64 bit - the solution may work on 32 bit or on Vista).
Why this feature - question with answer of which none is marked as accepted have note promoting accepting answers. Also the % of accepted answers is shown for each user. However such marking would say "in general this is good answer but I've tested it and it does not work for me" as oppose to "I'm lazy and don't read answers or accepting them".