Great question!

There's actually a bit debate over why some species prefer one or the other, or why some utilize both.

For asexual reproduction, there are a few benefits! For starters, you don't need to find a mate. If you're in the ocean all alone, it may be a long time before you end up encountering another of your kind. Similarly, in situations where you are in a position of high advantage, such as finding a large stock of resources, being able to quickly multiply is a tremendous advantage, too. Sex is costly in resources and time.

In the evolution game, asexual reproduction can be favored as it passes on 100% of parental DNA, where sexual reproduction passes on only 50%.

So, in light of all that, why should anyone sexually reproduce?

Sexual reproduction offers advantages within a shifting environment. If your body contains a deleterious mutation, for example, asexual reproduction makes you have that mutation every time until you die out. Sexual reproduction can give you a chance to cross chromosomes, swap genes for your child as a mix between parents, and potentially allow offspring that no longer carry that mutation, as they are now more fit. In this way, sex allows us to be purified, in a sense.

In an "arms race" scenario, it also allows rapid adaptation (evolution as an expression of the rapid change towards a particular variant in the population) and promotes diversity in general, priming us for rapid evolutionary change, where asexual beings are much more stagnant in evolutionary terms.

For jellyfish, they are versatile, allowing the benefits of both methods.