Superman needs to be Clark Kent as that's his only tie to humanity, Superman's greatest quality is his humility and Clark Kent helps with that. Also Clark Kent can help in ways that Superman can't by reporting on issues around Metropolis. Superman can't be Superman all the time, he needs to take a break sometimes. Can you imagine the stress that comes with hearing people all around the world call for help and knowing that you can only save a few of them? Clark Kent is where he goes to rest and Superman in most adaptations likes to be Clark Kent.
Being a reporter these days is not a good career path to be in. The hours are long, the pain is lousy, and your boss is always asking for better pictures of spiderman. Superman could switch to any number of better careers and be better off as a result, though he should probably only look into part-time jobs because of that whole superhero thing. Besides, what idiot actually can't tell that Clark Kent is actually Superman just because he takes his glasses off and does one thing different with his hair?
Superman need not interact with people at all. People need human interaction to share feelings and feel connected. As an alien, he knows that he can never fit in. For all I care, he can be Superman the entire time and watch over the planet from his solitude place that way he won't be distracted by all things human. I guess they have Clark Kent to show that even a geeky journalist can be a superman.
Superman cannot be Clark Kent. As you can clearly see Clark Kent wears glasses while superman does not.
A number of people have argued however that Clark would simply remove his glasses when "turning into" superman. This cannot be true aswell because after taking off his glasses Clark Kent now turned Superman wouldn't be able to see, thus greatly diminishing his combat abillities. Given that we know that Superman has an extremely supernormal ability to see he simply cannot be Clark Kent