I know there is nothing preventing me from publishing an article in a different research field, but is it beneficial other than for satisfying my own interest/curiosity? Presumably these may be added to my CV, and possibly included for funding applications?
I'm honestly surprised at the question. "Primary" does not mean "only"!
If it's a good paper, then of course you should publish it. Aside from the obvious personal benefits of making you look more diverse, increasing your visibility in another field, and possibly creating opportunities for future collaboration, you're doing what research is for: expanding human knowledge. And if it's not a good paper, then of course you shouldn't publish it, even if it is in your primary field.
But to echo shan23's comment: Don't use grants in area X to fund research in area Y.
I'd say it depends, on how tangential the research field is from your primary focus, and how you came to work on that field. Researchers working on interdisciplinary fields often have this kind of a scenario (which I would label as an advantageous one) - it would help you in case you ever wanted to change your research focus, as the grant agencies could see from your publication track record that you have done publishable research on more than one research field.
But, it might not be a very good idea to use funds/grants from your primary research focus for working on a different field. You should be able to show that these publications were in addition to the work that you published in your principal research area.
This is an interesting question. I recently was interviewed for a job and one of the interviewers asked me about a couple of papers which were clearly outside of my field of specialization. She wanted to know 'what these papers say about me and my professional capacity'. I was not prepared... and I said something along the lines of that ... I observe everything and try to identify problems and give solutions, even in a field that I am not directly studying... which, by the way, is my true reason to venture outside of my field. Now, looking at some of the responses maybe I was not wrong to go out of my field and produce some publications.