To my mind, footrest positions are in the same camp as bar risers, the hardness (or otherwise) of seats and not least, crash helmets and tyres. What suits one bod does not suit another. The why’s and wherefore’s as to why will vary.
Yes they are pricey but they do seem popular for whatever reason. Buy them and try them. If they suit you, great. If they don’t sell them, you won’t lose much.