I often find that "Search Q&A" is quite useless when I want to find a question or something related to it. But I've also noticed that when I'm about to ask a question, the "Questions that may already have your answer" box has many relevant results.
For example, see this search: failure of uniqueness Cauchy problem. It gives me exactly zero results. Now if you paste the same request into Ask Question title box, you'll get lots of results (including the one I was initially looking for).
So, is it really by design? Does Search Q&A have some different use than what I supposed it does? Wouldn't it be a good idea to at least make the duplicate-finding functionality easier to find since it works better at finding questions by a topic given in free form?