What are some fields of computer science where your stuff has to actually work? I am currently a PhD student in computational social science, and I feel like my work doesn't really contribute positively to humanity.  Basically I feel like I am running analyses and producing plots to "prove a point" or (optimistically) discover new insights about the world.  But I'm not actually sure that the point is true, because if I run the analysis in a slightly different way, it proves another point entirely.  It seems to very much depend on the specific dataset you use and the specific way you conduct your analyses.

I want to switch to a field where this doesn't happen.  Ideally in order to have a publication you would have to produce a piece of software that does something useful for the world, and it would be impossible to fake its usefulness.  I am affiliated with the computer science department at my university, so I was wondering what fields of computer science are the most "legitimate" in this way, and also friendly to people with very little background in them.
I work in the field of computational social science and during my PhD, there were many instances in which I had similar thoughts.

You provide me a hint in your second paragraph - you are affiliated with the CS department in your university. Unfortunately, a common fallacy of most CS departments which have programs in computational social science is to focus on the "computational" part and not really on the "social science" part. It is not an uncommon workflow to use machine learning techniques on a dataset collected using other computational techniques and then report results. This is because of many reasons, the least of which is that folks need to know computation in great detail - this is at many times possible in computer science departments and not in social sciences (although that is changing in many departments that I know of)

This is what is probably causing the disconnect in your mind. I strongly suggest either expanding your reading to the parts of social science that you work on or are interested in and talking to more social scientists. You need to think at the intersection of both computational and social sciences and not as a computer scientist. Social science contributes many understandings of individual and collective behavior (often in networks) For instance, there is a big difference in paper writing and reporting of results in WWW, KDD versus CSCW and CHI although all these venues promote and support computational social science.

Ultimately, at the end of the day, if you feel like you need to change your field you probably should! You should do what excites you the most but I just wanted to point out possible avenues of thinking about computational social science since it is quite novel.