14 DAY TRIAL // It doesn't wait for you in its slippers, holding a baseball bat while aggressively approaching you, don't worry. It couldn't do that, it's an email service, not a wife substitute. Nevertheless, the more it knows about you the better it is for you and a better and more organized life you'll have.
At the moment, the only use for the option to add a secondary email address to your Google Account is that it automatically adds the notifications you receive there in the Calendar service. And on the same level of importance, although not exactly the same deal, is that you can then use that additional email address to sign in to the Google Account instead of using the username or Gmail address.
Be it the case that you have two separate addresses from Google's mail service, one for your personal life and one for business, you can't just add the second and thus link them together, as you'll get a notification that "A user with the email you specified already exists." Great job, Dr. Gmail Watson! Of course it exists, it's also me!
The process for adding the second email address is quite simple in design, you go to your Account page and click the "Edit" button next to your "Personal Information" and, on the page you'll be redirected to, you'll be able to see something very similar to the screenshot on the left. You fill that in and an email will be sent to that address for verification. It's that easy and you can add more than one address.
Now, back to the comparison to the wife that wants to know everything about your life when you're not with her, who you're with, where you're at and what you're doing. Is this the first ever reported such case? I mean yeah, sure, there's functionality but it's also prying, if you'll let it come to that. Is that what you want?