Used for the temp sensor, and part of the bypass if I'm not mistake. It's going to be a pita to get to unless you remove the plenum. Easy otherwise, you do have to buy the preformed shape for replacement. If you do not, the hose kinks and will not work properly. Shouldn't be much more than $20 or so from the dealer.

There is also the bypass that runs under the intake manifold, if you are getting down to it, good idea to just buy both upper and lower intake gaskets, (genuine toyota preferred), the little "rainbow" hose as you called it, and take the plenum and intake off to take a good look at what's happening. If the bypass is leaking, you will kick yourself for not fixing it the first time. The bypass does not use any gaskets if I remember correctly, just silicone, so make sure you have a tube of that hanging around also.

Here is a link Cooling System

If you look at the top of the diagram you will see that hose you are talking about. Just to the right, see the arrow pointing down into the bypass? That's a metal pipe that exists under the intake, if you remove the plenum it will be more apparent, and not much more labor at all to fix it. Just label all of your vaccuum hoses, take pictures, whatever, but after that mess of hoses, it's just a matter of removing nuts/bolts, and parts. There is a sequence to removing the intake manifold, look at the fsm at the top of this forum for more info......Have fun