"...so once you breed it back you can produce a stable hybrid?"
Not exactly. Stabilizing a hybrid (dehybridizing it) involves primarily selfing the hybrid, and not back-crossing it to either parent. However, all is fair in hobby plant breeding, just like in love and war. But dehybridizing is primarily a forward breeding thing, of saving seeds each generation from the plants that most closely resemble the good hybrid that you are trying to stabilize. If, in a successive generation, you find that you have several different plants that are pretty close to the "target", then it might be helpful to intercross them. But in no case are you trying to get back to the original species of Zinnia violacea, which is a rather weedy looking single purple wildflower.
"My questions are mainly to understand why all the breeding if your hybrids can easily be lost or revert back to species."
The primary answer to "why all the breeding" is that it is fun. There is a certain enjoyable anticipation waiting for a bloom from any mixed packet of zinnias to open up, to see what the flower will look like. The suspense is even more enjoyable when the zinnias are from hybrid seeds of your own making. The hybrids aren't easily lost. In fact, all of the commercial zinnias are either stabilized dehybridized hybrids, field-mix bee pollinated zinnias, many of which are hybrids that even the bees don't know the parents of, or commercially produced F1 hybrids from known carefully inbred proprietary strains. I doubt that any of us have ever seen an original species Z. violacea (sometimes referred to as Z. elegans), and even if we did, we probably wouldn't recognize it as a zinnia. It is kind of an urban myth that hybrid zinnias "revert" to the wild species. What actually happens is a complex recombination of genes, so that the results are best thought of as recombinants. They aren't reverting, they are recombining characteristics.
I will discuss the subject of "ploidy" in another message. It is hard for me not to get "long winded" when discussing zinnias. I am attaching a picture of a bicolor zinnia selected from Veseys Zig Zag strain. I like bicolors, and many of my hybrids involve bicolors.