2. Its pretty much convention but sometimes we want to distinguish between "row vectors" and "column vectors" because we want to treat them as matrices. We can multiply $\begin{pmatrix}a & b & c\end{pmatrix}\begin{pmatrix}d \\ e\\ f\end{pmatrix}$ or $\begin{pmatrix}a \\ b \\ c\end{pmatrix}\begin{pmatrix}d & e & f\end{pmatrix}$ but we cannot multiply $\begin{pmatrix}a \\ b \\ c\end{pmatrix}\begin{pmatrix}d \\ e\\ f\end{pmatrix}$ or $\begin{pmatrix}a & b & c\end{pmatrix}\begin{pmatrix}d & e & f\end{pmatrix}$.