doubt on fsuid

[sysabod]

Hi all;
i saw a book said this below about file system id

In highly specialized circumstances,a program may want to keep its root permissions for everything but file-system access,for which it would rather use a user's uid.

My doubt is if a program could not even gain file system access,what can it do ? Isn't file-system access the most basic requirement during the life cycle of a program

well,if the book is right(very likely),what is specifically the so-called highly-specialized circumstances ?

Thanks in advance !

[cran]

well, if those files belong to another user, then wouldn't that be incorrect to allow the program to just retrieve anything file wants, even if it can access other programs? Plus, I think it would allow tracking for file creating too, otherwise all the files you'd create would belong to root instead of the user creating them... I'm tired too, so if anything is incorrect... well, I'm going to sleep.