What does "This question is left for future research" imply?
The existing answers are good.  Let me just add one more phenomenom I've seen in my area: it is not uncommon for the "future research" section to contain ideas that the authors don't plan to follow up on, but they think might be interesting.  Maybe they hope to inspire other researchers.  Maybe they're just curious, but don't have time to continue to pursue those questions.

Occasionally, the future work section is used by authors to respond to criticism that "you should have done experiment X" from reviewers.  Adding a sentence to the future work section is an easy way to respond to the reviewer and be able to claim you've acted on their comment in some way: it lets the authors respond to the reviewer comment with something like: "our future work section makes clear that we didn't do experiment X and we consider it out of scope for this paper, but we agree it would be interesting, and we've added it to the future work section".  You can form your own opinion about whether you think this is a good phenomenom or not; I'm just reporting on what I've seen.  Sometimes I've even seen authors include this kind of statement in the future work section of a submitted paper, as a preemptive innoculation against comments the authors anticipate getting from reviewers.  I don't know whether it is really effective, but you can keep this in mind when you read future work sections.  Occasionally this additional perspective may help you understand better why sometimes stuff gets written in the future work section.