lightroom is good for cataloging, like balto said, but its two most important uses, imo is the ability to very easily batch edit, and the fact that you can nondestructively edit raw files, without altering the original file.
in my workflow, i pull a batch of photos into lightroom, do basic editing on them (exposure, clarity, vibrance, and a few other things) and then export tiff versions of what i want to finalize into photoshop where i cant set the black and white points, do local edits and finalize for prints.
If you are just using your photos for web things its probably ok to just edit with lightroom. i did it for awhile when i had my old laptop abroad with just lightroom to update my blog. but to really get everything that is possible out of your photos you need to be using photoshop as well. it is just capable of a lot more than lightroom - especially if you are making prints, paper profiling, easy color correction, finely tunable sharpening are all things that are a must that you will have to manipulate in photoshop.