Why do shell script comparisons often use x$VAR = xyes?

if test "x$SHELL_VAR" = "xyes"; then
...

if test $SHELL_VAR = "yes"; then
...

if test $SHELL_VAR = yes; then        -->  if test = yes; then
if test x$SHELL_VAR = xyes; then      -->  if test x = xyes; then

if test "x$SHELL_VAR" = "xyes"; then  -->  if test "x" = "xyes"; then

if test "$1" = "abc" ; then ...

if [ "$1" = "abc" ]; then ...

if [ -z "$SOME_VAR" ]; then
  echo "this variable is not defined"
fi

SHELLVAR=$(true)
if test $SHELLVAR  = "yes" ; then echo "yep" ; fi

# bash: test: =: unary operator expected

if test $UNDEFINEDED = "yes" ; then echo "yep" ; fi
# bash: test: =: unary operator expected

SHELLVAR=" hello"
if test $SHELLVAR = "hello" ; then echo "yep" ; fi
# yep

SHELLVAR=" hello"
if test "$SHELLVAR" = "hello" ; then echo "yep" ; fi
#<no output>

SHELLVAR=" hello"
if test "x$SHELLVAR" = "xhello" ; then echo "yep" ; fi