Let's say someone is wondering about something, do a quick search and find nothing relevant, so post his question. Then some people point out that the answer already exists on some other question (and they are right!), like in Does a magic item used as an improvised weapon count as a magic weapon? the OP wants to know about whether magic items count as magic weapon too and the answer is in two other questions that wouldn't be found for a search like "improvised magic weapon".
So now the guy who posted the question got his answer, nice! But other people who will look for the same question will find the guy's question, with no answer but links to the duplicates. That's not that bad: they just have to follow the links. In some case however it can become quite bothering when you have to follow a link and then look through all the questions to find the one that is answering your problem. And then you can't really ask for more precision as the answer can be very old and you could even be off-topic.
Of course, on the example I linked the problem is very minor (there are only two answers on each questions), but I remember to have seen on stackoverflow (not rpg.se) links that send you to another question which is also closed as a duplicate, which send you to another one with more than 100 answers... etc. Obviously we are not going to have any problem of that scale, but shouldn't we try to promote new questions that have a better title instead of old ones that won't be stumbled upon by most of people?