Sometimes an E-mail is used on other company systems as a log-in, in whole or in part. I used to sympathize with past co-workers who had names much longer than mine, imagining them having to type "really_long_first.really_long_last@company.com" just to log into some web site.

I've also seen IT people enforce their random rules on everyone, leading to things like "rba186" as someone's actual E-mail address name instead of something meaningful.

So yes, choosing your own log-in and E-mail name would be pretty nice.