Justme’s answer is spot-on. The end termination on the other side of the differential pair wires is never perfect so the series termination provides extra attenuation of any reflections sent backwards from the end termination.
There’s one more important thing to note, though. When you terminate both ends of a transmission line, it gains a special property: a single impedance discontinuity in the middle of the line results only in a reduction of amplitude at the receiver, never ringing or overshoot. The idea is that any negative reflection sent backwards from the impedance discontinuity toward the transmitter dies at the source termination and doesn’t bounce back toward the receiver.
Imagine you wanted to build a system with two boards and a connector in between to pass some PCIe lanes in between. The connector between the boards represents at least a bit of an impedance discontinuity, so the both-ends termination is great here because it lets you go at fast without (as much) fear of the ringing and reflections that would inevitably result from driving a non-both-end terminated line at high speed.