Article III of the US Constitution requires that federal courts deals with "case or controversy" only -- not engage in theoretical debates. So federal courts must have a plaintiff (who must have standing to sue, in civil cases) and and a defendant.
In Taiwan, most of the time 司法院大法官會議 (rarely 司法院大法官審理, which speaks volume) has no litigation party before it, and its decision 可大可小 in terms of scope, and vague.