From practical experience from using jj daily and having (disposable) mega merges:

When I have discrete, separate units of work, but some may not merge soon (or ever), being able to use mega merges is so amazing.

For example, I have some branch that has an experimental mock-data-pipeline thingy. I have yet to devote the time to convince my colleagues to merge it. But I use it.

Meanwhile, I could be working on two distinct things that can merge separately, but I would like to use Thing A while also testing Thing B, but ALSO have my experimental things merged in.

Simply run `jj new A B C`. Now I have it all.

Because jj's conflict resolution is fundamentally better, and rebases are painless, this workflow is natural and simple to use as a tool