In which pratical cases are used parallel and thevenin terminations?
Proper termination is needed at the beginning and end of a transmission line.

If the termination is not there or has the wrong impedance, the signal you're trying to transport will reflect and distort. This becomes an issue when the length of the transmission line is longer than the wavelength of the signal.

Termination is not always of the Thevenin variant. It depends what the next circuit, which is receiving the signal, needs. If that is a simple inverter then a DC voltage of 0 V (ground level) will not make it work. Inverters need around half the supply voltage to act as an amplifier. This is where the Thevenin termination comes in. It terminates the transmission line but at the same time provides the proper DC biasing voltage for the next stage.