Honestly, different horses (and riders) have different requirements. The responses give a pretty clear idea why it could be beneficial to post. Some horses that are inclined towards tightness or hollowness in the back can benefit from posting through the lateral exercises at points. Likewise, I have also found that a few horses really struggle with rhythm - posting can help them tremendously.
Adding to the kettle, some people ride certain movements better in posting vs sitting (or vice versa). This can be an entire combination of factors - rider build, athleticism, old injuries, horse movement/suspension, who carries tension where, etc. I am hilariously terrible at doing a leg yield in a posting trot. It is a hot mess. I am significantly improved when I sit. It is because I have a weaker side that loses efficacy when I post, vs in the sitting trot. And vice versa, some riders (and caveat: when I talk about issues like mine, and this one too, they are very likely not a professional’s situation - but for amateur riders for whom horses aren’t a career or a profession, regardless of how capable we are I think we have more baggage to work through just because we have fewer miles to pick up good habits) have a tendency to really curl up in the sitting trot and their leg wanders too far in front or behind them so they lose the ability to really use it effectively. Posting can really benefit these people (especially when learning new exercises or working on known exercises that are challenging for them).