Why do we add "%" when we create node?

You don't have to if you don't want to. ;-) The % is used for route domain formatting where you may have the same IP defined in two separate locations. The % allows you to then segregate that IP to a route domain ID.

Here's some background info: https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/tmos-ip-routing-administration-11-2-0/2.html