It’s handy to have multiple email addresses if you have multiple contexts in which you interact with people.  You could create separate mailboxes, but that both prevents you from mixing emails if you want to, and requires extra administration.

It’s easy to configure this with Mac Mail in Yosemite.  In the “email address” field on Accounts | Account Information, just separate multiple email addresses you want as possible return addresses with commas.

You’ll then get a dropdown list of choices for from which email address you want to send when you email.  Of course, you’ll have to make sure that those addresses are actually forward to your email box by creating email aliases with your mail server, or using a catch-all.