What will Lightroom do that photoshop won't do?
Technically, nothing. I'm sure you could, with extensive experience and a thorough knowledge of PS, recreate all the effects that LR gives you. However, it would require a great deal of manipulation and time compared to just moving a bar in LR. In addition, LR will very easily allow you to upload your images directly to the internet as well as a module that will create photobooks (a really cool addition). But there is something that PS won't do and that's to sort and categorize your photos and allows you to sort them very quickly.
I think you should download the free trial and check it out. I've never heard anyone complain about LR, it's a product that's made specifically for photographers and once you get used to it, you're hooked.