Aside from just personal taste, keys can be very expensive. So you can get several keyless flutes in different keys for the cost of a similar one with keys. Details vary with the maker and it seems to be hard to find online price lists just at the moment (probably due to the messed-up trade situation). But you can go a long ways with instruments in C, D, and either F or G (depending on what kind of music you plan).
Also, flutes in alternate keys can be used to address awkward range issues (e.g. tunes that drop below D). It can also be helpful to own the small and large versions of certain keys (notably C or D). The large instruments are sweeter but the small ones are quite a lot louder and easier for fast tunes.