My question is pretty straightforward. FIA tyres and wheels regulation states:
Pirelli nominate two mandatory sets for each car for the race (which can be of different compounds) and one further set of whichever is the softest compound that can only be used in the Q3 segment of qualifying, but the teams are free to choose the remaining 10 sets.
I understand the second part of the rule. It makes sure that the most important qualifying session is Q3, the last one. Better for the fans, as you can most of the times just watch Q3 for the fastest laps.
But I do not get the purpose of having two mandatory compounds during the race. What is the point of this rule?