I'd admire the functions of sympy library, what it does is really awesome. The integration functions are really few guys intellectual work. But where exactly we use them practically on computers? If not sympy, do we really need such libraries? If we need them, what languages would satisfy our needs?
In some scientific fields (from experience: mostly mathematics and physics; most engineers prefer to use numerical approximations), often is interesting to calculate exact forms for integrals, derivatives, solutions of differential equations etc...
Thus, SymPy becomes useful for those applications. Also, I imagine it being useful for educational applications (where an exact answer, at least for simple problems, is interesting)