Which bit of the air defence role was employed when going fast down low?
The majority, at least during exercises, when the vast majority of targets were low and fast. Helps the missile in a front shot and essential to chase down the targets and get rear aspect shots with IR and gun.
And in offensive ops essential as an embedded escort or sweep for aircraft such as the GR. Nothing pissed off a low level attack formation than a medium level escort like a big finger in the sky saying "they're over here!!".
In that scenario they could also open the taps and extend ahead of the bomber package to engage an identified fighter threat ahead.
I know the B-1s appreciated them being tasked as escort rather than F-15s.