If I spend half an hour researching an answer I don't want to later find out that it had already been answered, on another thread, before I even started looking for the information. Knowing that a post is a duplicate thread can be very useful under such circumstances!
happens so often now that it would take up so much time... but it is daft when you go into Twitching and Birdwatching and someone is asking for the capital of some obscure Eastern European nation or the name of a philosopher who believed the earth was a giant cricket ball or similar