I'm not smart enough to understand why, but it DOES help.
I've found with Disney attractions, there is a large difference in the volumes used. The best example I can think of is EEA, where during the initial movie you might want to pick up the deafening explosion from the Big Bang yet capture the subtle noises in the jungle afterwards. Without a battery pack for your mics, you might either rely on AGC (big no-no) or have to adjust the levels while you're recordig (another big no-no).
The 1.5v coming out of the mic jack just doesn't give you the ability to reproduce attractions accurately. My battery pack provides 9V (maybe it's 10 due to the capacitors) and does great. I also use my recorder as an audio source for my videos and have been very happy with the results. I've done a few weddings (not professionally, just for broke friends) and not having the mic overload during applause, etc have been a life saver (and makes the brides happy even though they don't realize it).
Do you need a pattery pack? No... But you will wonder how you got along without one once you do. I found the price tough to justify, which is why I made my own, but if you are serious about live recordings, then they are a must for me. They DO NOT work for induction mics and ALD recordings.
Some recorders mic sockets come with power - the Zoom H2 will be Ok
I was typing while you answered.. Yes, mic power is probably there, but not sufficient in many cases. As I mentioned, there's a HUGE difference between 1.5v and 9v.