I would say the actual 'significance' of the slip rings and brushes is their distinction from the split ring commutator that you find on the dc generator, which is the more complicated one. Unlike the ac generator, which has two slip rings that both stay connected to the same ends of the coil as it rotates, the dc generator has a single split ring. Every half-turn of the coil in a dc generator, the two ends of the coil switch which half of the split ring they are connected to as they rotate. The induced pd in the coil still reverses every half-turn, but the split ring basically cancels this out (whenever the induced pd reverses, the connection between the sides of the coil and the halves of the split-ring are also swapped) and causes the induced current in the circuit to always travel in the same direction.
If an exam question actually asked you to explain the significance of the slip rings and brushes, I don't think you would have to explain the split ring commutator as well, but highlighting exactly what the ac generator does differently is important.