Honestly, the Pope is not allowed to teach error in faith and morals. However, when great crises have hit the Church, councils have been called. Part of the reason is that, even though the Pope cannot teach falsehood, that doesn’t keep people from misrepresenting what he says. In addition, a Council represents the fullness of the Church, and has the ability of uniting the bishops with the Pope. Why is this important? First of all, by keeping the individual bishops from heretical teaching themselves. It’s said that at the Council of Nicaea, the bishops were astounded to see how much of a hold Arianism had on the Church.
In addition, by having a Council, the bishops can go back home and say, “This is what WE believe, not just what the Pope is telling us that we must believe”. Popes have generally held their fellow bishops in great esteem, as fellow shepherds of the Church. Sure, they could have always lorded it over them and said, “I can’t teach error - you must believe what I tell you” every time. But honestly, that shows no respect for the office of bishop, which is the fullness of orders. Instead, by following Peter’s example and allowing for debate at a Council, the Pope fosters unity over disunity. As bad as the Great Schism was for unity in the Church - not to mention the Reformation - imagine how divided the Church would be if the Pope just issued “ex cathedra” decrees every time there was a controversy of belief. The only diocese that would still be faithful to the Pope would be his own! It’s not surprising that the “ex cathedra” power of the Pope has been used extremely rarely (other than for beatifications and canonizations) - the last time it was used outside of beatifications and canonizations was to fully declare the Immaculate Conception as a tenet of the Catholic faith.