A: When it’s a shock, even though it has a spring installed over it (aka “coil-over”)
If you’re unsure, a strut is used as a suspension locating device in place of the upper control arm; it is load bearing for lateral forces just like a control arm. A strut weighs more than double what a shock weighs, exclusive of the weight of the spring, as it must sustain large lateral forces, whereas a shock experiences zero lateral force, only force along its axis.
Not to be confused with a simple coil spring over shock. Struts weigh two to three times as much as a shock. A good comparison is the Honda Civic, sixth gen (shocks and upper control arms) vs. seventh gen (McPherson strut). See photos attached…
What are the performance advantages of a strut? Cost to manufacture is about the only one…
From a motorsport perspective, we can do cool stuff with struts but bottom line is a McPherson strut was born of the need to make something of acceptable quality whilst lowering cost to manufacture.
In other words, you’ll never see a strut on a purpose built race car constructed with a reasonable budget that’s not rules limited in suspension design.