I think we are talking of two different things, I was talking of the extension document/specification, that one seems to bring no particular innovation.
The actual implementation (by SEGGER or by someone else):
>Q: Can I implement BigFAT myself?
>A: Absolutely. BigFAT is a specification made available by SEGGER. Anybody is free to write a piece of software implementing it. No fees, no royalties, no headaches. You do not even have to let SEGGER or anybody else know.
is what may allow that (random access), this implementation would be useful if - instead of a "feature" of a given app/program - it would be implemented as a filesystem driver of sorts.