[derefr]

Presuming charitably that you mean the more interesting question of "why is this a program rather than a thing other programs do internally when they realize they need elevation": well, two reasons:

1. any program can call fopen(2) and fwrite(2), and yet cat(1) exists. Unix plumbing is mostly there for cases where you're linking programs together in ways those programs didn't expect.

2. Privilege separation. You don't want big, complex programs running as root. You want big, complex programs running as your user, speaking to tiny little well-hardened programs running as root over a pipe, where the tiny-little program can only do one thing.

For example, you might have seen the pattern of piping things into `sudo tee [file owned by root]` in order to be able to write to a file that's owned by root. This fits both of the above considerations: moving the privilege into "tee" rather than having whatever command is generating the text, exposes less of a vulnerability surface; and also, it's `sudo tee` rather than tee(1) itself performing elevation, because tee(1) itself was written a decade or two before this pattern emerged, and so has no idea it could be used this way.