Math.SE cannot afford to be a place for everything math(s)-related: that way lie endless ‘proofs’ of the Goldbach conjecture, ‘demonstrations’ that Cantor was wrong-wrong-wrongity-wrong and $\Bbb R$ is really countable, and questions about the best diet for doing non-commutative algebra.
I’ve no doubt that for some users Math.SE is first and foremost a place to do mathematics, asking questions as they arise and helping others with theirs when they can; indeed, I’ve spoken with such users in Chat. Moreover, one can see some evidence of this in the available user data: one user, for example, has asked $106$ questions and answered $114$; another, $94$ and $122$. And of course as they arise is flexible enough to cover the possibility that for some users they simply don’t arise, so in some sense this description could fit just about anyone here. However, it’s very clear from the user pages that for many users Math.SE is primarily a place to ask questions, and for many others it’s primarily a place to answer them. Here, for instance, are some statistics from the first page of users sorted by all-time reputation:
$$\begin{array}{cccc} \text{Rep in thousands}&\text{Nr. of Users}&\text{Answers}&\text{Questions}\\ \hline 150\text{+}&3&14380&2\\ 100\text{ - }150&2&4795&55\\ 70\text{ - }100&6&15469&114\\ 50\text{ - }70&7&8981&77\\ 30\text{ - }50&18&18296&284\\ \hline \text{front page}&36&61918&532 \end{array}$$
Of those $36$ people six have asked no question, $17$ have asked fewer than five each, and one person accounts for a little over $20\%$ of the questions asked by these people.
For me Math.SE is primarily a place to teach mathematics, and the data above suggest that I’m not alone in this. The doing is part of the teaching and often contributes to my enjoyment, depending on the problem, but producing what I think is a good explanation or hint brings a pleasure that is independent of the mathematics involved. It is secondarily a place for me to learn mathematics: sometimes in order to answer a question, occasionally from another answer to a question that I’ve answered, and once in a while from an answer to a question that simply caught my fancy.
Given both the Question & Answer format of the site and its explicit openness to questions at all levels, it’s inevitable that teaching (as distinct from providing more or less collegial assistance) will be seen as one of its major functions, both by those who want to teach and by those in search of teaching/tutoring. And in practice teaching is one of its major functions: answering this questionthis question is teaching every bit as much as answering this onethis one, this onethis one, or this onethis one is. (I might add that it’s by far the easiest place to teach on-line that I’ve seen: its technical platform makes it especially attractive to those of us who have taught on-line in straight-ASCII environments like the Usenet groups alt.algebra.help and alt.math.undergrad, the Topology Q+A Board, or the Math Forum @ Drexel.) That is, your second possibility describes not only my view of what Math.SE ought to be, but also what it actually is.
Finally, I don’t think that your first possibility really exists. One could of course establish and enforce rules that would reduce the scope of Math.SE by greatly reducing the amount of explicit teaching, but the result would in my opinion no longer be a place to do mathematics at all levels: it would substantially favor the higher levels. Math.SE would become a sort of Junior MO.