The admin guide simply says
It’s possible to configure Nextcloud to distribute the data over multiple buckets for scalability purposes.
Does scalability mean, it’s faster than using a single bucket?
I don’t think speed is the major factor - I would rather expect the limiting factor on the Nextcloud side (much heavier workload vs simple storage) I could imagine it’s more to overcome buckets limitation e.g. number of files or maybe size of the files stored in a bucket.