1, because the larger the input impedance, the signal source can basically all fall on the amplifier, so as not to be consumed by the internal resistance of the signal source.
2, under a certain voltage, the larger the output current, the stronger the load capacity. The larger the output current, the smaller the resistance in the output circuit
The greater the input resistance, the smaller the current provided by the signal, which can amplify a weaker signal. The smaller the output resistance, the larger the output current, and the stronger the load capacity.
For example, if you amplify a signal source voltage, the voltage itself will be divided by the internal resistance, so the larger the load resistance of the amplifier circuit as a signal source, the more you can ignore its internal resistance, and your amplified output is also equivalent to the signal source, for the latter stage of the circuit the smaller the internal resistance, the better