According to the Bash Reference Manual, it's about convenience.
The Advanced Bash Scripting Guide has a more detailed explanation:
"A builtin is a command contained within the Bash tool set, literally built in. This is either for performance reasons -- builtins execute faster than external commands, which usually require forking off 1 a separate process -- or because a particular builtin needs direct access to the shell internals."
Also note that echo does exist as a standalone utility on some systems. Here's what I have on my Darwin system (MacOSX 10.5.8 - Leopard)
$ uname -a
Darwin host.foo.org 9.8.0 Darwin Kernel Version 9.8.0: Wed Jul 15 16:55:01 PDT 2009; root:xnu-1228.15.4~1/RELEASE_I386 i386
$ bash --version
GNU bash, version 3.2.17(1)-release (i386-apple-darwin9.0)
Copyright (C) 2005 Free Software Foundation, Inc.
$ which echo
/bin/echo
echo is also available as a builtin, but apparently my scripts use /bin/echo on my Mac, and use a Bash builtin on most of my Linux & FreeBSD systems. But that doesn't seem to matter, because the scripts still work fine everywhere.