Battery Grip for DSLR?

I'm into a lot of landscape and wildlife photography. I currently have a nikon d90. I was wondering if I should purchase a battery grip to prolong the use of my camera with minimal battery changes. When I'm out in the field. But I was wondering if it is actually necessary to buy one for this camera.

Yes, you can get a battery grip if you like. It will provide you with 2 times the battery life, and a vertical shutter button for shooting in portrait orientation.

It is always better to have a backup battery. If the battery grip really helps, then get one.

I have found link for you: http://www.hootoo.com/dslr-battery-grips-c-383_372.html.

How would it prolong the use of your battery?

When one battery runs out, you just put in a second one.

Is that really any more time consuming then putting two batteries in from the start?

There's an advantage to not using a grip. In cold weather you can actually "recharge" a battery by putting it in your pocket and warming it back up.

The only time a grip is really useful is if you are shooting in a country where power is not readily available, but the style of grip will allow you to use AA batteries that are available, or for people who want to add a vertical grip for ease of shooting.