IMAP servers often strongly recommend prefer clients to which report their ID, for usage statistics and bug reports. Clients may can also find use server IDs useful for the same reasons. Although the RFC requires otherwise, some servers do make it have a soft requirement that the ID is reported (and will sometimes even disconnect clients they do not recognize). See https://tools.ietf.org/html/rfc2971 for more details. See also the github PR at https://github.com/ruby/ruby/pull/2882.