An example I can think of immediately is a requirements document that references the fuel loading tables in the specifications for an aircraft. Another example is referencing other MIL-STD documents that give very detailed information on, for example, military imaging formats. That same goes for RFC documents. (I still use MIL-STD-498 for documentation when it is up to me what we use.) A lot of time, developers need information that is outside the scope of the particular document are writing and including references to detailed information elsewhere is invaluable.