Why do I need Lightroom if I have Photoshop?

I'm not sure what the word 'premier' means in this context. If you mean that you already have/use Adobe Premiere, then the answer is simple. Adobe Premiere is a video editor, Lightroom is an still image organiser and editor. Apples and oranges.

If you mean you use Adobe Photoshop, then the answer is a bit more complicated. You don't absolutely need Lightroom if you already use Photoshop, but the using the two together is what most photographers do. They use Lightroom to manage their images and do the basic editing of their raw files. In most cases that is enough. But if you want to go further and want to do things like compositing, then you switch to Photoshop.