In a standard config, Junos requires that the next-hop IP address is directly reachable, which is what you would want normally. If the next hop is not directly reachable you can configure Junos to do an additional lookup and see if it has a route to a non direct next hop, this is known as a recursive lookup where the system looks in the routing table twice, and is configured using the resolve option. There are other vendors that do this by default, but it is not optimal to do multiple queries to the routing table instead of just one. Junos requires you to configure it manually to make sure that is your intent, as it would degrade performance.The resolve option will look in inet.0 and inet.3 for the indirect next-hop, the resolve knob is the only thing needed to enable it.
I have used it under two different situations. In an old situation with dial on demand routing where links would only be up temporarily for modem and ISDN connections. The other time was in a VPN network, where once again a connection and routes were not always present. I am sure there are a few other use cases.hope that helps