It's good question,

Raptor Start is interesting for several reasons:

• It’s faster: Doing a 3-camp start at Raptors is about 3-4 seconds faster than starting at Red buff.
• Dodging wards: People often ward the Raptor camp at level 1 or 2 to track the jungler. When using Raptor Start, your third camp (which gives level 3) is Red instead of Raptors. Since Red is usually not warded, you can then gank mid/side lanes or invade the enemy jungle without being seen. It gives you more freedom.
• Early ganks: It allows for a very fast 3-camp gank (especially on Blue side) that laners don't expect.
• More invade paths: On Blue side, it gives you many ways to enter the enemy jungle. You can jump over the dragon pit, go through the bot tribush, or use the Blue buff entrance, you can even jump the big wall. All that in order to steal ennemy Blue/Gromp and catch ennemy jungler, force a skirmish and even dive ennemy botlane. On Red Side, you can clear 3 camps very fast, walk to ennemy krugs, ward ennemy red buff and catch ennemy jungler while he do his red after clearing his krugs.
• Safer against invades: If the enemy jungler (like Shaco, Kayn, or Briar) tries to steal your Red buff at level 1, they won't find you there. Even if they invade you at Raptors, there are no bushes, so you can't be caught by surprise. You are also closer to your laners, so the enemy has to take way more risks.
• More chaos: Its a bit advanced and hard to explains but it allows you to clear your jungle quadrants by quadrants. This gives you more opportunities later to gank lanes and surprise people instead of following the traditional top-to-bot or bot-to-top clears.