If you log onto lexus.com and register your vehicle (free) it will provide you with an owners manual (via acrobat) or you can send a request to get one for free. If you don't want to do that here are some answers to your questions:
1. safety feature, if you're zooming down the freeway and you're in H, your center of gravity is too high, thus a potential rollover, L is used primarily to ease the egress for passengers and for loading purposes. If you want to stay in the selected mode, select your desired mode the turn "OFF" the automatic feature while in that mode.
2. In extreme situations, stuck in mud, snow etc.
3. If in N, there is no power to your wheels at all. H for higher speeds, L for lower speeds, climbing steep hills, towing.
Hope this helps