"Why are wheelie bins half-buried in parts of Luxembourg?" has four close-votes now and will likely be closed as off-topic soon.
I was about to cast the fifth vote, but then I thought about it a little bit.
I thought it was a very interesting question. However, it's certainly not very much related to travel. Well, OP saw it during traveling. But that would also mean that people could also ask about almost anything curious they see, anywhere in the world? That is very broad. That would mean this could almost turn into "ask us anything" Yahoo Answers. That may be an exaggeration, but you know what I'm saying. (
No offense to Yahoo Answers.)
So while I didn't vote to close, I don't feel bad if it's closed either. IMO it might be better if OP asks this at Home Improvement SE.
However, Jonathan Reez pointed out in the comments that if we're gonna close that one, we should also close the popular question "Why is this bathroom symbol in Germany “00”?" One may argue that this question about a toilet in a brewery is more travel related because tourists are more likely to visit breweries and toilets than they are to get involved with underground [sic] trash cans. I guess that's fair enough. And the "00" question is also quite interesting.
Next, I myself commented that by this standard, the popular question "What does “K+M+B 2016” mean in when written on a church wall?" might also have to be closed. Another quite interesting question. Also, one could argue that it's far more common for travelers to visit churches than to be concerned with trash cans. Religious buildings are, after all, a chief tourist attraction in a vast number of places. However, this is also a matter of taste. When I travel, I like to just stroll around the city/town and look at what life is like there. I would have also probably noticed the trash cans myself and become curious about them. Thus, both can be said to be similarly travel related. And where the trash can question could have been asked at Home Improvement, the church question could have been asked at Christianity SE. (Not that that matters much, though. But personally, I would have asked both questions at those respective sites.)
What the church question has going for it is that it can help you understand the culture of the country. The toilet question, much less, but still to some small degree, I suppose? As for the trash can, I'm not sure. Seems miniscule, and not very much related to culture.
In either case, how can we be consistent here? Is the trash can question really that different from the other two?
IMO we need some clearly defined criteria. Do we have them? Otherwise, how can we be fair when deciding to close or leave open?