Spacers are used to represent the physical spacer objects placed between dies in a die stack. The spacer provides separation between the two die, be it for electrical/thermal/etc. reasons or to ensure there is adequate vertical spacing so that, when the upper die is mounted above the lower die, the lower die's bond wires will not be damaged.
Interposers contain routed clines and bond fingers, normally. The fingers provide a point for bond wires coming from dies mounted above the interposer to connect to. The routing clines then connect these fingers (normally located closer to the center of the interposer) to a second set of fingers (normally around the periphery of the interposer). A second set of bond wires will connect from these peripheral fingers down to the package substrate or to another die/interposer lower down in the stack. This helps to reduce the length of bond wires (and amount of gold/copper needed for the wires) and minimizes the length of the span of the wire over a lower die in the stack that might extend a significant distance out from the edge of the die mounted above.
When you use these items will depend upon your specific flow and design requirements, however. I can't tell you when you will add them to your design. All I can say is that the more accurate your design, the more accurate the SI extraction, 3D view (and 3D bond wire DRC checks), etc. will be.