I would argue that the lion is, or should be, important to us because it is a living, unique species. It is part of a food chain and an ecosystem on Earth that we don't fully understand. Would you take a running automobile and tear the gas tank or the right wheel off and ask if we should keep or get rid of it? It needs all of these pieces to function properly, as does the environment of our planet. I would argue that we should never willingly, knowingly and deliberately participate in causing a species to go extinct, or watch while it happens if changing how we live or act can somehow help.
The lion may get more attention than other species also in danger because it is so recognizable, like the polar bear or the spotted owl. I'm sure there is a Desert Prairie Rat or something that may also be going extinct, but that would be hard to get people to rally around because that species isn't as big or popular a symbol as the polar bear or the lion is.
Lastly, as to why we should keep it on Earth, I would say that I don't believe it is our choice to keep or remove it. It is life, and life is sacred.
Lion is a big strong animal from the cat family. If we think of lions in isolation, there are no specific sue of lions for human beings. But when we think of lions as a part of the nature and the eco-system within this nature, lion is an important part and a link in the eco-system. Thus lion is important to the humans for maintaining the eco-systems of which lions are generally a part.
Of course, lions do form some attraction for humans in zoos, circuses, and in wild animal sanctuaries. But, it is important that humans recognize lions as some thing valuable in itself and preserve them for their intrinsic value rather than for some materialistic benefits received from lions.