Sightstone gives you free wards every time you go back, but also precious stats, especialy early in the game. An early sightstone on the jungler gives your team a huge vision advantage, and makes him tankier for a cheap cost.
The jungler also has more option for placing wards, since early on your support will probably be limited to botlane and the surounding jungle: if his team is winning, your jungler will place aggressive wards in the enemy jungle to be able to counter gank and snowball even further ahead.
when your team is losing, he will ward his own jungle to allow his team to come and help him when the enemy jungler is invading, thus allowing the team to get back into the game.
You'll notice that in pro games and most high elo games, it's usual for the junglers, and even most laners, to buy 2-3 wards on each back, to assure vision for their team. Buying a sightstone on the jungler allows him to keep doing so, while not delaying his items too much, since junglers are usually starved on farm later in the game.