Why do you jungle? What is the point in it? And is it worth having a team mate 2v1 from the beginning?

Several reasons...

One of the big ones is the jungle provides quite a good bit of XP and Gold. You dont want that xp/gold to just go to waste, or worse go to your opponents do you? Good teams don't typically give away dragon for free, why would they do the same for the jungle.

Also it gives a teammate the ability to solo a lane. While the xp isn't as good as it used to be they can typically expect to hit 6 a lvl or 2 ahead of duo lanes. Thats pretty good. Also they get more last hits than they would if they had to share the lane with someone else.

So not only are you getting a comparible amount of xp/gold as you would in a lane, but you also give your lanemate slightly more xp and almost 2x the gold.

Also it opens up the opportunity to ganking lanes as you are frequently mia.

Teams can win without jungling, but they typically go through and milk the jungle when their lane is pushed to still get some gold/xp from it.

being MIA also makes zoning easier for mid lane. Even if your not going to get a middle gank, you can appear in mid often to harass either landing damaging hits, or just ensure your mid maintains lane control Leveling faster than anyone in game.

Usually mid lane doesnt mind you coming to farm minions if your jungling fast. The sooner you get your gold the sooner you have the items you need to stack your hero to your personal build. Don't underestimate the power of enstilling fear.