Both the outbound interface and next hop address can be set for a static route. Whether to set these parameters depends on specific conditions. In fact, the next hop address must be specified for all routes. This is because when a packet is sent, the route to the destination address of the packet is searched for in the routing table. The link layer can find the corresponding address at the link layer and then forward the packet only after the next hop address is specified. 

•  For a point-to-point interface, the designation of the outbound interface indicates that the next hop address is designated. The next hop address of the route is the address of the peer interface that connects to this interface. For example, when POS packets are encapsulated with PPP, the IP address of the peer is obtained through PPP negotiation. In this case, you only need to specify the outbound interface. A next hop address can be specified or not specified.
•  For NBMA interfaces that support point-to-multipoint networks, in addition to configuring the IP route, you need to configure the secondary route (mapping between the IP address and the address of the link layer) at the link layer. In this case, the IP address of the next hop should be configured.
•  The next hop must be specified for the Ethernet interface. This is because the Ethernet interface is a broadcast interface. This may result in multiple next hops and thus the next hop cannot be uniquely determined. In this case, if the outbound interface must be set to a broadcast interface (such as Ethernet interface), or an NBMA interface, the corresponding next hop address of the outbound interface must be specified.

    The static route that is configured with only the next hop needs to go through next hop iteration before participating in routing. If the iteration is successful, the static route can participate in routing. Otherwise, the static route cannot be preferentially selected. The static route that is configured with both the next hop and outbound interface can directly participate in routing. Only the routes that are preferentially selected during routing can be added to the FIB to guide packet forwarding.

Good post.