Why is there a /bin/echo and why would I want to use it?

If you open up a bash prompt and type in an echo command, that uses a shell builtin rather than running /bin/echo. The reasons it is still important for /bin/echo to exist are:

1. You're not always using a shell. Under a variety of circumstances, you run an executable directly and not through a shell.
2. At least in theory, some shells don't have an echo builtin. This is not actually required.

To expand on #1, suppose you wanted to move all the regular files whose names started with abc anywhere in src to dest. There are several ways to do that but one of them is:

find src -name 'abc*' -type f -exec mv -nv {} dest/ \;

But suppose, rather than just running that, you want to see every command that will be run first. Well, then you can prepend echo to the command, just as you might in other contexts:

find src -name 'abc*' -type f -exec echo mv -nv {} dest/ \;

But find doesn't use a shell. That runs /bin/echo.

Besides find with -exec or -execdir, the /bin/echo executable will be called by other programs that themselves run programs but not through a shell. This happens with the xargs command (which is related to find), as well as in a number of other contexts, such as the Exec= line of a .desktop file. Another example is when you run sudo echo, which can be handy for testing if sudo is working.

Similarly, some shells have a printf builtin but /usr/bin/printf also exists.

A less common possible reason you might deliberately use /bin/echo is if you were relying on the differences between it and the echo command provided by your shell. man echo documents /bin/echo; help echo in bash documents the bash builtin. echo is not very portable, because different implementations--both across operating systems and across shells on the same operating system--support different options (e.g., -e) and differ in their treatment of backslashes. Of course, it's better to avoid relying on such details, and use printf instead, which is far more portable.

In bash, you can make the type builtin show /bin/echo as well--assuming /bin is in your $PATH as it always should be--by passing it the -a flag:

$ type -a echo
echo is a shell builtin
echo is /bin/echo

Eliah's done a great job of answering this, but I want to comment about the "why is there another version of echo separate from the Bash program" part. That's the wrong question.

The right question is: why is this a builtin in the first place, when it could have been (and is) a perfectly fine external command?

For simplicity, take a look at the builtins in dash, a measly 38 (bash has 61, for comparison, going by the output of compgen -b):

.               continue        getopts         readonly        type
:               echo            hash            return          ulimit
[               eval            jobs            set             umask
alias           exec            kill            shift           unalias
bg              exit            local           test            unset
break           export          printf          times           wait
cd              false           pwd             trap
command         fg              read            true

How many of these need to be builtins? [, echo, false, printf, pwd, test, and true don't need to be builtins: They don't do anything that only a builtin can do (affect or obtain shell state that's not available to external commands). Bash's printf at least takes advantage of being a builtin: printf -v var saves the output to the variable var. time in bash is also special: by being a keyword, you can time arbitrary command lists in bash (dash doesn't have a time equivalent). pwd doesn't need to be a builtin either - any external command is going to inherit the current working directory (and it's an external command too). : is an exception - you need a NOP, and : is it. The rest do actions that an external command can easily do.

So, a fifth of these builtins don't need to be builtins. Why, then? The dash manpage* actually explains in passing why these are builtins (emphasis mine):

Builtins
 This section lists the builtin commands which are builtin because they
 need to perform some operation that can't be performed by a separate
 process.  In addition to these, there are several other commands that may
 be builtin for efficiency (e.g.  printf(1), echo(1), test(1), etc).

That's pretty much it: these builtins are there because they're used so often, interactively and in scripts, and their functionality is simple enough, that the shell can do the job. And so it happens: some (most?) shells took on the job.** Go back to the sh from 2.9 BSD, and you won't find an echo builtin.

So, it's entirely possible a minimal shell can skip implementing such commands as builtins (I don't think any current shell does, though). The GNU coreutils project doesn't assume that you're going to run them in a particular shell, and POSIX requires these commands. So, coreutils provides these anyway, and skips those which don't have any meaning outside of the shell.

* This is almost identical to the corresponding manpage text for the Almquist shell, which is what dash, the Debian Almquist shell, is based on.

** zsh takes this idea to the extreme: the commands you get by loading various modules, like zmv, are things you wouldn't think a shell need even get into. At that point, the real question is: why would you use bash instead of zsh, which has all these builtins?