Why are there only limits and colimits? Part of my intuition about the construction of limits and colimits is based on the idea that they are initial and terminal objects in the appropriate category: The limit of a diagram $D$ is of course a/the final object in the category of cones over $D$, and similarly for colimits and cocones. The part that I don't understand is that there seem to be four options here, and only two are commonly discussed:
$$\begin{array}{c|cc}
 & \text{Terminal} & \text{Initial} \\
\hline \text{Cones} & \text{Limit} & ?? \\
\text{Cocones} & ?? &\text{Colimit}
\end{array}$$
What fills in the blanks, and why are they less emphasized? It seems like we want final objects when arrows are naturally going 'out of something' and initial objects when they're going 'into something', which vaguely makes sense but doesn't feel satisfactory. Products are of course important. On the other hand, nothing prevents one from asking for an initial object in some category of spans, but books (as far as I've read, which only involves the basics) never mention them. Is there a reason?

Here's one of the several equivalent ways of thinking about limits and colimits in which this question doesn't arise: for a diagram $J$ and a category $C$, there is a natural diagonal functor $C \to C^J$. If limits of shape $J$ always exist in $C$, they organize themselves into a right adjoint for this functor, and if colimits of shape $J$ always exist in $C$, they organize themselves into a left adjoint for this functor. And that's it. 
