GI very frequently requires you to do either hospitalist, hepatology, or chief year in order to get in because of how competitive it is (almost, not always). Cards doesn't always require it and is usually more reasonable but very frequently a chief year can boost your chances of getting into cards. Also keep in mind that, just as with residency, some fellowship programs are a LOT better than others, and in cardiology, typically people don't just stay as general noninvasive cardiologists - they'll go into interventional or EP. Getting into an even better cards program is sometimes worth the extra chief year.
Also, cards and GI are the most competitive overall, but many of the "less competitive" fellowships have different tiers of competitiveness within the fellowships - especially heme/onc, critical care, and ID. Some programs are far and above better than others.
For what it's worth, I've been to programs where of, say, 5 chiefs, 3 were planning to go into a hospitalist career. They didn't want to dive right into it and were basically getting paid while having a relatively low workload* compared to clinical duties, all the while being able to apply for jobs, go and interview, got more time to study for the IM boards, and catch up on family raising and all that good stuff. It also teaches a LOT about hospital management, managing a team, and generally being a "boss", which is something you don't get a lot of training or education about during medical school. One chief I talked to wanted to do it because he wanted to join a teaching institution and ultimately do a lot of administrative stuff, teaching, etc. and it made sense to agree to doing a chief year.
*(Not to say that a chief year isn't a lot of hard work otherwise, just different kind of work)