There are several reasons we should cite sources (in both questions and answers):
- Although many questions can be answered without context, many others cannot! And usually, it's the people writing the answers who can tell whether or not they need context, not the askers.
- Even if a question can be answered without context, it's often harder to answer.
- And if you quote a source in an answer, it could be considered plagiarism if you don't tell us where it came from.
Recently I've been participating in ELU and ELL, and on those sites people are generally required to cite their sources. On the English sites, posting a quote without a source in a question may get it closed, and posting a quote without a source in an answer may get it edited or deleted. I don't see why Japanese.SE should be different in that respect.
So please, if you quote something, whether it's in a question or an answer, tell us where the quote came from!