[Politically Correct]

She need this large group for the cover up.
Remember, although she erased the server, all those emails went to other people on other servers and still exist.
I expect that some of the more damaging ones will surface later in the campaign.

[justlurking]

Not all of them.

The real reason for the server: people with accounts on the server could communicate among themselves without any record, except on the devices they use.

Wipe the server and those devices, and those emails disappear.