Sponges can reproduce sexually and asexually, but avoid self-fertilization. What is an advantage of avoiding self-fertilization? Why would budding an exact clone be better?

Self fertilization will produce offspring that will all be genetically similar to the parent sponge. Having all offspring and parent having the same genetic make up can be bad in an unfavorable condition in which if they are not able to adapt will ultimately result in these organisms been wiped out.

Avoiding self fertilization ensures that genetic materials and characteristics are properly mixed within the sponge species, equipping the offspring with higher chances of adaptability and survivability to varying envoirments.

An exact clone might be better in the case in which the sorrounding envoirment is highly favorable to the original parent sponge. In this case the clone offspring will have all the better chances of surviving and thriving in the same favorable envoirment than if a new gene is added.