I'd probably answer that in two ways.
One is that not every GAAR case applies in the way that the Crown hopes, so sometimes it's necessary to make these amendments when the GAAR is found not to apply. That also happens when in a lot of cases when the government spots a new type of planning that's being used to exploit the current rules.
Amendments are made before the general anti-avoidance rule has been applied and litigated, because that kind of case can take many years to be assessed and work its way through the courts, and during that time, there's a lot of uncertainty for taxpayers. What often happens is the government makes these amendments in order to, in colloquial terms, close the loophole immediately to provide the certainty up front, and then the cases can be challenged later in court.