Like other mentioned here, jungling has a few benefits:
- getting more experience globally for the team if the lane you left can still get exp without being zoned out or denied
- getting more gold if the lane you left can still get last hits
- the enemy does not see you unless they warded the jungle
It as well weaken the lane and can expose you to ganks if you do not ward properly.
One of the most important factor for deciding if you can jungle is to know whether you have to go back to fountain with this hero and whether you can farm fast enough to keep up.
Some heroes can jungle because they have summons or can control/vaporize creeps to soak the damage for them (Nature), others because they can lifesteal (Naix) or reduce the incoming damage/are tanky (Axe), or a combination of both (Doom).
Lifestealer can survive in the jungle although he will be sometimes on lower health making him vulnerable to a gank if your jungle was warded. The reason he gets more powerful in the games you have seen is only that he is one of these really strong late game carries if not ganked and left to farm. You are not expected to get more exp than a dual lane in early levels by jungling, although when you get midas you can use it to on the stronger creeps to get exp faster.
Enigma can farm quite fast in jungle and Axe needs a bit of luck but he does very fine in jungle ...
When you jungle, you should always take in account the opportunity cost vs laning.