The benefits are primarily that it looks good (to most people), it is a readily-available meal for various grazing species (ie., tangs), and it can sequester nitrates and toxins. Also, they introduce dissolved oxygen through photosynthesis, and compete with less desirable microalgae species. The potential drawbacks are that some species may have periodic die-offs, especially if their growth goes unchecked or if iron is in short supply. If this happens they sporalate, and that can deplete dissolved oxygen. Also, if it gets too weedy it can compete with corals for lighting. I have it and like it. It came on my live rock and took off once I bought some proper lighting (4w/gal). It isn't necessary; it is more a matter of taste. I like some greenery. So does my purple tang.