One reason to know back substitution is that it is relevant when doing numerical mathematics.
A standard procedure to numerically solve linear systems $Ax =b$ especially if one wants to solve for more then one $b$ (which is very common) is to perform an $LU$-decomposition of $A$ (or something similar like a Cholesky decomposition), that is one decomposes $A$ as a product of a lower triangular matrix $L$ and an upper triangualar matrix $U$ (sometimes one needs permutation matrices in addition but let us ignore this detail).
Then, one solves the two triangular systems $Ly=b$ and $Ux=y$, via back substitution, which is very 'cheap' relative to solving a general linear system ($O(n^2)$ vs $O(n^3)$).
One might now ask why not solve the triangular systems via Gauss--Jordan elimination, and I do not fully oversee momentarily if there is even a difference depending on how one sets things up precisely. Still the two things, reducing to triangular systems and solving the triangular systems, are conceptually quite different, and that this difference is stressed when one uses back substitution could be considered as an advantage.