Why do people use: if [ "x$FOO" = "x" ] .... ?

[Bill Stewart]

	I've seen a lot of Bourne shell scripts that test for
blank variables using the construct
	if [ "x$FOO" = "x" ]

Why the x's?  If you leave out the quotes, then the test dies if $FOO
is empty, but why not just use
	if [ "$FOO" = "" ]
or
	if [ -z "$FOO" ]

I've seen this construct in new code as well as old, from reputable
programmers.  I've checked a few obscure cases like
	FOO='`echo You Lose!!`'
but even they don't die.  Is this just another superstition, like
	sync; sync; sync;
or am I forgetting something stupid and obvious?
			Thanks;
				Bill Stewart, AT&T Bell Labs, Holmdel NJ
				{ihnp4,allegra,cbosg}!ho95c!wcs