I'll try to explain this in a simple way
Zsrc is the output impedance of the previous stage.
You can think of impedances like frequency dependant resistors.
A filters transfer function is calculated by evaluating the complex impedance for every component in the filter, and you end up with one equation for the transfer function, that describes the filters impedance.
As you can see, the filter (which is now represented as one big frequency dependent resistor) sits in parallel with the source. With a little source transformation it becomes apparent, that the source impedance and filter impedance forms a voltage divider.
So there you have it. That's why the source impedance matters.
In general: The smaller your source impedance is, the less it will influence the signal being transferred to the next stage.
Plotting the right vaue in Zsrc's place requires you to calculate the previous stages output impedance.