This other taxon will be better known, usually bigger, and will continue to exist, even after the segregate taxon has been split off.
A segregate will be either new or ephemeral: there is a tendency for taxonomists to disagree on segregates, and later workers often reunite a segregate with the 'mother' taxon.
[1] If a segregate is generally accepted as a 'good' taxon it ceases to be a segregate.
Thus, this is a way of indicating change in the taxonomic status.
It should not be confused with, for example, the subdivision of a genus into subgenera.