There isn't always a helmet sitting there, but if a fielding team wish to have fielders close in to catch the ball, the helmet is there for their protection. If the situation arises where the fielding team don't need fielders in a close catching position, therefore not needing the helmet, it is normally placed behind the stumps, behind where the wicketkeeper stands. There is a 5 run penalty against the fielding side if the ball hits it, so it is generally accepted that behind the wicketkeeper is safest