Why in op-amp inverting circuits one of the op-amp inputs is grounded?
The circuit you show is called an inverting amplifier.
The non-inverting input is grounded in this topology to the same ground as the input signal.
An op-amp has very large gain, so any voltage differential on the inputs will cause the output voltage to change so as to minimize the input voltage differential due to the feedback provided by Rf.
That will drive the difference between the + and - inputs to very close to zero.
So the input voltage almost entirely appears across Rin, causing a current of Vin/Rin to flow in Rin.  That current almost entirely flows through Rf, since an op-amp's inputs are very high impedance.
That leads to the output voltage being -Vin/Rin*Rf.
Other topologies do not have one of the inputs grounded.  For example a non-inverting amplifier or a differential amplifier.  (Google will show you lots of examples.)