Long-time occasional browser, first-time poster here. I recently asked How much did it cost to attend events at the ancient Roman colosseum? and have gotten (so far) two answers, upvoted by the community, that provide information but don't present sources. Is this level of sourcing to be expected? I don't want to sound ungrateful and I'm definitely not complaining; I'm just trying to understand community norms. (I also understand that the question is new and this is a weekend.) How do I know if an unsourced answer from an anonymous person on the Internet is correct -- votes?
Community moderated site, so you get to help determine the answer. Personally I strongly prefer sourced answers. Sometimes I fall short of my own goal, but I think sources are important.
Sources help to avoid perpetrating history myths. History is, in general, more complicated than what we (I) learned in school. School teaches a simple mythology where Washington chopped down cherry trees and colonists shot redcoats out of the woods. Reality is more complicated, and sources force us to go beyond the simple.
As @Colyn1337 says, sources help when the answer sparks enough interest to spur further research. Sometimes I read an answer that is intriguing. Sometimes I read an answer that is wrong. Either way, the source helps me to understand.
Sources are invaluable in reconciling apparent conflict. Several people have pointed out that I have a ... certain bias... with respect to economic questions. Sources enable others to reconcile my answers with those of others (whose bias is perhaps more mainstream). I'm not wrong, but the sources help to put my answers in the correct context.
History is about assembling narrative from sources; if we omit sources, I'm not entirely sure we're practicing history.
I support your request for sources; you are part of the community and we collectively determine the conventions/standards for our community. (Sorry to be trite, but this is something I believe in).
I think sources should be added so interested parties can do further research. Otherwise history SE will become the SE version of yahoo answers.
As the guy with the mop, I will say that I frequently find myself needing to delete heavily negatively-voted answers that have no sources listed or linked, and multiple comments and/or flags complaining about lack of references.
So just on the superficial objective level, I can tell you that good answers here tend to have linked references in them for non-trivial assertions, and bad ones tend not to. A person who wants to maximize their upvotes and reputation would be well-advised to take that as a model.
Personally, I will generally not upvote any answer with no sources whatsoever, except in exceptional circumstances. Exceptional circumstances occur though. For instance, I'll confess on that particular question I (mis?)used an answer upvote as a 10pt hug to a longtime user that I'm tickled to see actively participating again. (Hi Rose!)
I would suggest that sources are desirable but not always essential or possible. For one thing, sources/historians disagree - sometimes violently. I was one of those who answered the question mentioned, but only stated that I understood X to be the case (and could be wrong!) I suspect most if not all of us "know" things from long ago studies and half-remembered reading. To insist on sources for every answer would, IMO, waste the collective expertise, knowledge and wisdom of the community. It is always possible and acceptable to ask for sources for a specific reply, but I think to insist that every assertion is referenced would severely reduce the number of responses. Good question nonetheless.
Edit One further point - a lot of "references" are to Wikipedia, which is just that - a Wiki source. It is not written on tablets of stone and, if I were writing an academic paper, I might use it as a starting point, but would certainly not rely on it
Maybe it's just my imagination but there seems to have been an increase in the proportion of unsourced answers (which is why I'm 'resurrecting' this question). Actually, I could have picked any one of several questions on meta to do this (including this one which I answered) so some people might be thinking 'Oh no, not this again'.
Aside from commenting 'sources please' on answers without, there doesn't seem to be much we can do other than raise awareness and keep arguing the case for sources.
Personally, it's highly unlikely that I'll upvote an answer without sources, especially if the question is on something about which I know little. Why? Because without sources it becomes more difficult for me to inform myself and to assess the quality of the answer. If it's on something I know a fair amount about, I'll often be able to recognize where the information (probably) came from (but it should still be sourced for those who have no more than a superficial knowledge).
It is very unlikely I'll downvote answers for not citing sources but, if any new users are reading this, putting sources in your answers is much more likely to get you votes - and you'll soon be on your way to privileges.