Rootless containers are quite useful for things like accessing computing clusters which are shared and thus nobody gets root access on them (such as in universities). Rootless building is quite useful for other reasons (mainly because it allows you to build without ever hitting a privileged codepath which is arguably a huge security improvement), but to be honest it's more of a cool feature you can implement once you have the far more useful tools implemented (such as rootless storage management and rootless containers).