depends on how wide. small amount is pointless i guess and for larger spacers you MUST use hubcentric spacers or you'll got wobble/uneven tyre wear etc.
you will be slightly widening the track of the rear of the vehicle which in theory should allow faster cornering but in reality your just putting more strain on your wheel bolts for little to no gain.
get wider wheels, better tyres, better suspension or even anti foll bars. much better investment
imho