You might want to ask the Lightroom enthusiasts in the Lightroom forum http://forums.adobe.com/community/lightroom?view=discussions for more detailed arguments in favor of Lightroom.
I can tell you what I use it for and why I couldn't do without it:
1) image management: LR is a powerful database and image management tool. I don't have to worry about giving my photos descriptive names or saving them in particular folders. Instead, when I import them into LR, I give them keywords so I can immediately find them whenever I need them. The LR filter system is great, and you can create collections of any groups of photos you want.
2) backup: When you import photos into LR, you can designate a backup destination to backup the images. I backup onto an external server, so if my hard disk crashes, I still have my images. You can also define how often and where to backup the LR catalogue (which is the actual database)
3) non-destructive editing: the images you edit in LR actually remain untouched. The edits are merely recorded in the database, so you can always go back to the original version. If you select an image in LR to edit in Photoshop, you can opt to edit the original in PS or edit a copy. Once the image is edited in PS it is automatically visible in LR
4) virtual copies: since the original image is not touched, you can create different versions (virtual copies) of any image without taking up a lot of disk space.
5) editing capabilities: you can do all the editing in LR that you can in Camera Raw, and more. I use the synch function a lot: If you have a bunch of images that all need the same edits (removing color cast, adding clarity, removing noise, etc. etc.) you only need to edit one and synch the others and they all get the same makeover. Most of the time I don't have to go into PS at all anymore to improve photos.
The best thing for you to do to help you with your decision is to download the trial and view the available tutorials.
I find the history bar in LR, in Develop mode, to be quite useful. Camera raw in PS doesn't seem to have that, there is no visual history (that I have found yet). Once you get out of ACR and into PS there is visual history, but nothing in ACR.
One of the main reasons for using Lightroom or ACR (Adobe Camera Raw) in Photoshop is that it is a raw converter.
Is your Photoshop the full version, Photoshop CS5 or just Photoshop Elements?
Elements has a stripped down version of ACR. The full Photoshop and Lightroom use the same version of ACR with access to all the features.
When you shoot raw format such as NEF, CR2, etc. you are getting the FULL dynamic range data from the sensor.
When you shoot jpegs the camera automatically takes the sensor data, throws away a lot of data, crunches the rest down and uses its own guidelines to produce a jpg that it feels should be acceptable to the average user.
While you can open jpegs and tifs in Lightroom or ACR, you have less ability to edit the file without loss simply because you are dealing with a lot less data.to begin with.
Point and Shoot cameras almost never have the ability to shoot in a raw format.
A higher end cameras like a DSLR or Micro Four Thirds can shoot in raw (and jpegs at the same times if you so desire)
Lightroom/ACR can do different things than Photoshop and vice versa, Neither one is a replacement for the other.
What you should be clear about is that Lightroom is Photoshop's Camera Raw plugin (ACR), wrapped in a file organizer to make it a stand-alone application. That's basically all.
Lightroom is streamlined for high-volume production, but there's nothing you can't do in Photoshop/ACR. But for some purposes you can do it faster and more efficiently.
I had Lightroom on a 30 day trial, but turned it down. The deal-breaker for me was the "libraries"-concept - I just can't work that way. I need to have my files organized on disk, not in some abstract virtual catalog.
You can still have your files organized on disk if you want to, that's under your control when you import. Regardless of how you organize your files on disk, the ability to have robust filtering of your images via keywords and and the ability to have virtual collections of images is the real + for me.