Different applicants might have very different reasons for applying for the very major, prestigious fellowships in the UK.
Having a very specific research/scholarly focus for which the best program is a UK school. This is a relatively uncommon applicant, but there are some sub-fields in which UK schools tend to have different approaches. For internationally-focused study (development? public health?, ) being outside the US is in itself an important new approach.
Having an academic passion, considering the possibility further along of a PhD but not feeling ready yet to commit. A master’s degree would let you try out going in depth in the subject and explore it from several angles – to determine whether you want to continue and if so, in what direction. Very roughly speaking, UK master’s allow you to focus in a more interdisciplinary way than would a US masters. And, there is next to no funding for US master’s degrees.
This exploration can also be helpful for those who aren’t sure whether their path stays in academia or is taking them into a profession.
Having an academic passion which you haven’t felt fully able to explore as an undergraduate – either because you only recently discovered it or perhaps were distracted by requirements of another degree or professional credentialing. These fellowship foundations are open to applicants who want to study something just because it’s fun. You’d need to know enough about the subject you propose to be able to do well in graduate school, and to articulate what you’re thinking in the first place but you don’t have to be a major in the area or planning on it for a career. If you won one of these fellowships, you’d be fully-funded, well able to explore and experience with no sacrifice to yourself. Whatever you have in mind for a career can probably wait a year or two – and the prestige name of the fellowships can be helpful anywhere.
Your reasons might be along yet another line, or be a combination of the above. The fellowship committees don’t show any preference to one sort of candidate over another. All that seems important is that you deeply understand your reasons and can articulate your story as it relates to your fellowship plans.