There are two types of site-to-site vpn that can be configured on the SRX, policy based and route based.
Route based vpn use a tunnel interface, the st0 that you reference. This interface provides a termination point for the routing and nat features on the vpn. Route based vpn are generally prefered but can be a little more complex to setup. But if the network grows beyond a few sites they are utimately easier to grow and maintain.
http://kb.juniper.net/InfoCenter/index?page=content&id=TN108
Policy based vpn combine the routing and policy all together in the configuration and do not require any tunnel interface. They do not allow the use of nat. They are also specific in the connected networks on each side. There are fewer steps needed to set them up. And it can be easier to connect to a non-juniper remote end point.
http://kb.juniper.net/InfoCenter/index?page=content&id=TN107