It does nothing to help with quoting, if $var is broken then so is x$var. About half the usages I've found (and this is some pretty old code) quote the variables correctly.
The assumption for this use was var was a valid single word but could conflict with switches to [ / test or it could be the empty string. If var was already guaranteed to be [^[:space:][:punct:][:cntrl:]]+, then x would also have been unnecessary too.
