From an austerely scientific point of view, it is of secondary importance. Overall, sexual reproduction serves to create new mixtures of the existing gene pool. We primates share this destiny with most of our fellow creatures. The fact that, as a rule, only two individuals take part in the mixing procedure may be less mathematically convincing than the participation of many, but has apparently proven sufficient and ideal in evolutionary cost-benefit accounting. And fidelity actually stands in the way of this mixing process.
Parameciums form the unbeatable opposite pole to the concept of fidelity.
They don't even bother to produce eggs and seeds, but simply couple to each other and exchange their genes directly. In the process, they have managed to develop 23 different sex-like states of being. That does not mean that they have to hunch together in large groups in order to create a new paramecium, but it does expand their pool of potential partners considerably. Instead of choosing a single sexual partner for life, or a new one every once in a while from roughly 50 percent of its fellow parameciums, the droll protozoans can choose among a good 95 percent of their fellows for reproductive purposes - not just one out of two like us higher organisms. But it doesn't seem to have given them much of an advantage, or the Earth would be knee-deep in wonderfully adapted and happily mating parameciums.