Isn't that what stdio is for (fopen()/fread()/fwrite()/etc)? It performs buffering for you.
"And why bother with stdio (or its Plan 9 equivalent) when it's so easy to do the buffering faster yourself?"
For the same reason you'd use any abstraction: to keep your program simpler. Even if you consider it "so easy" there are plenty of opportunities there for off-by-one errors and buffer overflows that would disappear if you used stdio instead.
My earliest UNIX programming was in the mid-80s on a machine with a (luxurious!) 1.5MB of RAM. I definitely remember avoiding stdio when writing small utilities that I wanted to start up and run fast.
Also remember that back in those days echo was not a shell builtin. (Hell, back then the testing operator '[' wasn't even a builtin. Some UNIXes like OS/X still have a vestigial /bin/[ executable!) Programs like echo that ran from shell scripts constantly had to be coded to start up as fast as possible.
This includes linux, but it's in /usr/bin/[
Put echo on a diet, removing unnecessary use of stdio and getopt.
Before...
-r-xr-xr-x 1 root wheel 58636 Oct 28 05:16 /bin/echo
After...
-rwxr-xr-x 1 root wheel 12824 Nov 12 17:39 /usr/obj/usr/src/bin/echo/echo