When we interpret Scripture, or any document from the past, it can be useful to have background information. That's because writers generally leave many things unsaid. They are usually writing to or for an audience that has a common frame of reference, so they don't engage in lots of exposition. Rather, the audience is expected to bring supplementary information to the text.
Problem is, what was common knowledge for readers at a particular place and time may not be common knowledge for a modern reader. So it can be useful to have background knowledge to help a modern reader fill in the gaps.
That, however, raises the question of what counts as suitable background knowledge. For instance, when is a parallel truly parallel? Without attempting to be exhaustive, I'll briefly mention two controls that I use:
1. The ostensible background information needs to have a foothold in the a text. It is illicit to import an interpretative paradigm wholesale from outside the text. For instance, consider ufological interpretations of Ezekiel's theophanies. That "background" information belongs to a frame of reference that's extraneous to the world of the text. Based on watching Hollywood movies about flying saucers and extraterrestrials.
2. The ostensible background information needs to have a foothold in reality. On the issue of comparative mythology, for instance, I don't use mythology as a starting-point. Rather, I go behind mythology to ask what experience gave rise to the mythology. How did ancient people experience the natural world? What are the natural properties of snakes, mountains, rivers, and "gardens" that underlie the symbolism? That provides a more reliable transcultural basis for extrapolating from one text to another text.
To take a stock example, the four seasons are typically used as a metaphor for the human lifecycle. That's due to the relative universality of the seasons. Of course, if you live in equatorial Africa or Latin America, that may not be an accessible metaphor.
Another example is imagery drawn from human social life to provide theological metaphors. But because these are analogies, they require some modification. For instance, the Bible sometimes depicts heaven as a palace. The Father is the aging king. His youthful Son is the Crown Prince. There's a throne room with angelic courtiers and sentinels.
In real life, the palace guard exists to protect the royal family from hostile intruders. But, of course, God doesn't need anyone to protect him, so the function is reversed: they protect unwary intruders from stumbling into God's presence!