Leech gives you complete control over your downloads and full browser integration. You can queue, pause and resume downloads, download from password-protected servers and store your passwords in your system-wide, secure keychain. And you won't have to worry about downloads that were disrupted by a crashing browser ever again.
Sure, your browser can download. But does it let you easily accelerate downloads, limit bandwidth, download on a schedule, or execute rules to sort incoming downloaded files? Doubt it. And clearly, you're not going to be able to quit that RAM-consuming monster of a browser until all your downloads are done.