Why does the wort need to be cooled?

Just wondering why this step is necessary. Is it to prevent bacteria from growing or if the wort is too hot does it kill the yeast when you pitch it?

Both.

If you pitch your yeast right after the boil, you'll kill them. Anything above 150, actually will injure them. 70 is where most people want to pitch.

The reason we use ice water baths or wort chillers is because right after the boil the wort is open to infection which is a bad thing. The less exposure time, the lower the chance of infection.

And finally, using a chiller produces what's called a "cold break" where proteins are released from solution and can be left behind as you rack to your fermenter: this makes clearer beer.

There's a lot of information on chillers and wort handling practices on the forums and in the Home Brew talk wiki.

Ok thanks for the info. I used a ice water bath in the sink and stirred the wort gently to help it cool for about 10-5 min. The water I added to the wort to top off to 5 gal had a thin layer of ice on top so it was nice and cold. After checking the airlock this morning its bubbling away nicely!

If you pitch too hot, even if you don't kill the yeast you can cause a lot of off-flavors to be produced (esters).

One other reason is to minimize DMS production which can result in an off-flavour. Unless you already did a long boil and drove most of it off it will continue to be generated while the temp is greater than ~ 160F. This is only a concern if you are doing AG, extracts have already had most of the DMS driven off.

GT