[j-nsp] New to Juniper: loopback interface address for the re0 configuration group?

[v]

Hello,

I am new to networking and trying to understand the initial configuration on a Juniper MX router with dual REs:
http://www.juniper.net/techpubs/en_US/junos14.2/topics/task/configuration/routing-engine-dual-initial-configuration.html

Step 6 is the following:
" Set the loopback interface address for the re0 configuration group:
[edit groups]
root at host# set re0 interfaces lo0 unit 0 family inet address address/prefix-length"

Why do I need to give the re0 group a loopback interface address in the first place?
What address should I give to the loopback interface?

Regards,
v