It is used so that you don't offend another person. For example, I could say, "Do you not care about the environment? (I'm) Just asking." The question could seem mean; so by saying 'just asking,' the question no longer seems so rude. It can also be used like this: "Do you hate animals?" "No! Why did you say that? That's mean!" "Sorry. I was just asking. I wasn't trying to be mean." I know that this might not be the best translation, but 'just asking' is like saying "Mondai shika desu" in Japanese. Also, what Meena said is also true. What I'vre written is just another usage of the phrase.

只是随便问问

For what I think, it means that you are willing to accept any answer, without criticizing; sometimes we ask explanations of human behaviour because we disagree, here we are only asking, we are stating that our personal opinion, if any, is absent from the conversation.

means to open a conversation , something like that.

Just asking = I'm just asking. just = there is no particular reason for my doing so