What's the benefit of linking my iPhone with my iPad in iMessages and FaceTime?

So you can access messages and have people reach you on each of your devices. I'm not sure i understand the question in your second post. You give your email address to people so they can reach you and it's the same with an Apple ID. You're Apple ID is the email address you use for Apple services. You don't HAVE to use it for messages and FaceTime. You can use another email address or a mobile number if you have an iPhone.

Read more here:

https://www.apple.com/ios/messages/

http://www.apple.com/ios/facetime/

Presumably, people that you stay in touch with will know your email address anyway, correct? If you call them using any phone at all, cell phone, smart phone, land line ....they know your phone number, correct? The advantage of using the iPhone number is that it you can send SMS messages on the iPad if you enable text message forwarding - as long as you meet the requirements. An iPad cannot send SMS messages natively, since it is not a phone. Sending SMS messages allows you to text with Android and other regular "dumb" phones.