I never understood these "add a new line" deals.
If you're an existing customer... shouldn't you already have the number of lines you need?
Mom, dad, two kids... are you gonna add a fifth line just to get a free phone?
Did you give your kids a phone the moment they were born?
Some parents like to wait until their kid is a certain age before giving him/her a phone. Some wait until the start of their fall school term. I'm waiting until my kid starts 8th grade/is 13 years old.
Here's another scenario: 2 people on different carriers get married and want to consolidate phone plans to simplify and save money. At some point, they'll have kids (adopt?). The kid will need eventually need/want a phone.
That's when these "add a new line" deals come in handy.